Chordates and non chordates

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Table of Contents
Chapter Page No.
A Scheme of General Classification in the phylum Chordata …………………………………………………..2-9
Origin of Chordates ……………………………………………………………………………………………………………...10-51
Protozoa ………………………………………………………………………………………………………………….52-117

A Scheme of General Classification in the phylum Chordata
Paul R. Harding, Jr.
Hendrix College
Phylum, Chordata
Subphylum, Hemichordata- - - Balanoglossus, Saccoglossus (dolichoglossus)
Subphylum, Protochordata
Class, Cephalochordata- - - Branchiostoma (Amphioxus)
Class, Urochordata (Tunicata)
Order, Ascidiacea --- Sea squirts
Order, Thaliacea --- salpians
Order, Larvacea --- larvacians
Subphylum, Vertebrata (Craniata)
Branch, Agnatha (Monorhina)--- with single nostril and circular mouth without
jaws
Class, Ostracodermi --- armored fishes; extinct
Orders: Osteostraci, Heterostraci, Anaspida
Class, Cyclostomata --- Unarmored, cel-like
Order, Myxinoidea --- hagfishes
Order, Petromyzontia --- lampreys
Branch, Gnathostomata (Amphirhina) --- two nostrils and with jaws
Grade, Pisces --- fishlike gnathostomes
Class, Placodermi (Aphetohyoidea) --- armored fishes; extinct orders: Arthrodira,
Acanthodii, Antiarchi, Petalichthyida, Rhenanida, Palaeospondylia Class,
Chondrichthyes (elasmobranchii)--- Cartilaginous fishes with gill septa (separate
gill clefts)

THE PHYLUM CHORDATA
Order, Cladoselachii --- Cladoselanche; extinct
Order, Pleuracanthodii --- Pleuracanthus, extinct
Order, Selachii
Suborder, Squali---sharks
Suborder, Raji (Batoidea)---skates, rays, sawfish, guitar fish Order, Bradyodonti
---Ancestral to Holocephali. Extinct, Cochliodus; Psammodus
Order, Holocephali---chimeras
Class, Osteichthyes---bony fishes without separate gill clefts
Subclass, Choanichthyes---primitive bony fishes; nostrils connected to mouth
cavity order, Crossopterygii --- lobe –finned fishes; ancestral to Tetrapoda
Suborder, Actinistia Family, Coelacanthidae----Latimeria chalumnae (1938,
1952, 1953, 1954)
Suborder, Rhipidistia Families: Osteolepidae, Rhizodontidae, Urostheneidae,
Holoptychiidae, Terrasiidae.
Order, Dipnoi (Dipneusti) ----Lungfishes proper
Subclass, Acttionopterygii (Teleostomi)---ray-finned fishes; nostrils not connected
to mouth cavity
Superorders: Chondrostei, Holostei, Teleostei
Grade, Tetrapoda
ARKANSAS ACADEMY OF SCIENCE
Subgrade, Anaminota ---- without extraembryonic membranes
Class, Amphibia
Subclass, Stegocephalia---- skull and cheeks roofed with bony plates
Order, Labyrinthodontia----teeth with abundantly infolded dentine in the pulp
cavity; body armored with scales or plates; extinct order, Gymnophiona (Apoda) -
-- Caecilians

Subclass, Caudata (Urodela)----salamanders and newts
Subclass, Salientia (Anura)----Frogs and toads
Subgrade, Amniota----with extrambryonic membranes
Class, Reptilia Order, Squemata
Suborder, Lacertilia (Sauria)–Lizards
Suborder, Ophidia (Serpentes)---snakes
Order, Rhynchocephalia----Sphenodon, Champsosaurus
Order, Testudinata (Chelonia)---tortoises, terrapins, turtles.
Order, Crocodilia (Loricata)---crocodiles, alligators, gavials, caimans
Order, cotylosauria----Primitive extinct reptiles, closely resembling the most
primitive amphibians (Labyrinthodontia) and of great phylogenetic importance
as links with them. Example: Seymouria.
Order, Mosasauria----Extinct large aquatic reptiles with long snake-like scaly
bodies, a crocodile-like head, strong recurved teeth, and two pairs of flippers in
place of legs. Examples: Mosasaurus, Clidastes, Tylosaurus, etc.
Order, Ichthyosauria----Extinct fishlike reptiles with long tapering rostrum.
Examples: Ichthyosurus, Stenopterygius.
Order, Plesiosauria ----Extinct long necked aquatic reptiles with flippers in place
of legs. Examples: Elasmosaurus.
Order, Pterosauria-- Extinct flying reptiles, pterodactyls. Examples: Pteranodon,
Rhamphorhynchus.
Order, Thecodontia (Phytosauria)----Extinct crocodile–like reptiles with long
narrow Jaws, and nostrils close in front of the eyes rather than on the end of the
snout. Examples: Belodon, Mystriosuchus.
Order, Pelycosauria---Fin-back lizards; extinct. Examples: Dimetrodon,
Edaphosaurus. Ophiacodontidae (example: Ophiacodon) ancestral to mammals.

ARKANSAS ACADEMY OF SCIENCE
Order, Dinosauria--- Extinct generalized bird-like and lizard-like forms; contains
the largest, most grotesque and most variable reptiles.
Suborder, Ornithischia----with bird- like pelvis; bird beaked, duck billed, bird
footed, some ostrich like etc. Examples: Ornithomimus and Struthiomimus,
ostrich- like; Trachodon, duck-billed dinosaur; Cory-thosaurus, hooded duck bill
dinosaur; Stegosaurus, two rows of vertical plates on back, brain exceptionally
small, spinal cord with brachial and sacral enlargements; Triceratops, head with
three horns and the skull extended like a shield over the back and shoulders;
Ankylosaurus, armored.
Suborder, Saurischia----With lizard–like pelvis. Exemples: Tyrannosaurus,
Brontosaurus, Brachiosaurus, Diplodocus, Compsognathus.
Order, Therapsida---Extinct mammal–like reptiles. Example: Cynognathus.
Evolved from order Pelycosauria, but not ancestral to mammals.
Class, Aves ---- birds
Subclass, Archaeornithes---“Ancient birds” showing reptilian ancestry; extinct,
Examples: Archaeornis, Archaeopteryx.
Subclass, Neornithes---“New birds” Superorder, Odontormae---Ichthyornis;
extinct. Toothed; good flyer.
Superorder, Odontolcae--- Hesperornis; extinct. Also with true teeth in sockets; a
flightless (wing of numerous only) diver.
Superorder, Ratitae---Cursorial flightless birds with unkeeled sternum.
Examples; ostrich, rhea, emu, cassowary, kiwi, elephant bird, moa.
Superorder, Carinatae---Adapted for flight, sternum keeled. Great majority of
modern birds.

Class, Mammalia
Subclass, Prototheria --- egg-laying mammals.
Order, Monotremata ---- duckbill and spiny anteater.
Order, Multituberculata ---- an extinct group of doubtful position.
Subclass, Theria ---- viviparous mammals.
Infraclass, Metatheria (Didelphia)---viviparous mammals usually without
allantoic placenta.
Order, Marsupialia --- pouched mammals. Opossum, kangaroo, wombat, wallaby,
flying phalanger, koala, marsupial mole, bandicoot, marsupial mouse,
Tasmanian wolf, Tasmanian devil, tiger cat, etc.
Extinct orders of doubtful position: Pantotheria (also called Trituberculata ----
probably ancestral to later types of mammals). Triconodonta, Symmetrodonta.
Infrasclass, Eutheria (Monodelphia)----viviparous mammals with allantoic
placenta.
Superorder, Unguiculata --- clawed mammals. Sloths, pangolin or scaly anteater,
hairy anteater, rabbits, armadillos, aardvark, rodents, bats, carnivores,
insective ores, sealiions, seals, walruses, etc.
Superorder, Primates ---- mammals with nails
Order, Lemuroidea----lemurs. Ruffed lemur, mouse lemur, tree shrews, slow
loris, aye-aye (Chiromys), etc.
Order, Tarsioidea --- tarsiers
Order, Anthropoidea----anthropoids. Men, apes, monkeys.
Superorder, Ungulata --- hoofed mammals.
Order, Condylarthra----condylarths; extinct

THE PHYLUM CHORDATA
Order, Dinocerata ---- Uintatheres; extinct
Order, Sirenia--- dugongs and manatees. Large fishlike forms believed to have an
ungulate origin.
Order, Perissodactyla --- foot with an odd number of toes, each usually sheathed
in a cornified hoof. Asses, zebras, horses, tapirs, rhinoceroses, titanotheres,
chalicotheros.
Order, Artiodactyla----foot with even number of toes, each usually sheathed in a
cornified hoof. Pigs, hippopotamuses, and such ruminants as camels, chevrotians
(mouse deer), deer, elk, moose, antelope, giraffes, cattle, bison, ox, water buffalo,
cape buffalo, sheep, goats, musk ox, llama, etc.
Order, Proboscidea---number of toes odd or even, each with small nail – like hoof,
nose and upper lip combined into a long muscular proboscis. Elephants,
mastodons, mammoths.
Order, Hyracoidea --- four toes on fore limb, three on hind; superficially like
guinea pigs, but related to hoofed animals. Coneys, Procavia (Hyrax).
Superorder, Cetacea---- body superficially fishlike; fore limbs of broad and paddle
–like flippers with embedded digits and no claws; no hind limbs; tail ending in
two broad transverse fleshy flukes.
Order, Odontoceti---toothed whales. Sperm whale or chacalot, killer whale,
narwhal, porpoises, blackfish, beaked whale, pygmy sperm whale, etc.Homodont
dentition.
Order, Mysticeti---whale bone whales or baleen whales. No teeth mouth with
numerous parallel horney plates of “whale-bone” or “baleen” on sides of upper
jaw used to strain small animals from water. Right whale, rorqual, gray whale,
blue or sulphur-bottom whale, humpback whale etc.

Order, Archaeoceti----zeuglodont whales; extinct. Dentition heterodont.
THE PHYLUM CHORDATA
In preparing the system of general classification presented here, the writer has
acted under the adoption of a constructive policy, and has contrived to make no
radical departures from conventional methods. Indeed, it has been the writer’s
wish to improve or build further upon the better foundations of general
classification.
In order to provide an understanding of the manner of grouping, brief notes are
given with the names of most of the different main taxa. For clarity, synonymy
has been indicated.
In reviving such groupings as Ungulata and Unguiculata, the writer has wished
not to show any tendencies towards an artificial system. It will be noticed that
the chalicotheres (horse allies with clawlike terminal phalanges) have been
placed in the Ungulata. Dinosauria has been regarded by others as an artificial
taxonomic group, and in reviving its use as a definite taxon, the writer feels that
he is justified because Saurischia and Ornithischia are well related through their
general similarity of limps, ribs, vertebrae, and skulls.
LITERATURE CITED
Colbert, E. H. The Dinosaur Book. 2nd ed. 1951 Goodrich, E. S. Vertebrata
(Craniata). First Fascicle: Cyclostomes and Fishes. Part IX of Treatise on
Zoology edited by Sir Ray Lankester, 1909. Harmer, Hardman, Bridge, and
Boulenger. Fishes, Ascidians, etc. Vol. VII of The Cambridge Natural History
edited by S. F. Harmer and A. e. Shipley. 1904. Reprinted 1910.
Hegner, R. W. and K. A. Stiles. College zoology. 6th ed. 1951.
Hyman, L. H. Comparative Vertebrate Anatomy. 2nd ed. 1942.
Jordan, David Starr. Fishes. 1925.
Newman, H. H. The Phylum Chordata. 1939.

Parker, T. J. and W. A. Haswell. A Textbook of Zoology, Vol. II. 6th ed. Revised by
Otto Lowenstein and C. Forster-Cooper. 1951.
Pirsson, L. V. and C. Schuchert. A Textbook of Geology. Parts I and II. Physical
and Historical Geology, 2nd sd. 1920.
Romer, A. S. Man and the Vertebrates, 1941.
Schuchert, C. and C. O. Dunbar. A Textbook of Geology. Part II. Historical
Geology. 3rd ed. 1933.
Storer, T. I. General Zoology. 2nd ed. 1951.
Walter, H.E. and L.P. Sayles. Biology of the Vertebrates. 3rd ed. 1949.
Zittel, Karl A. von. Textbook of Paleontology. Vols. I, II and III. Edited,
translated and revised by Charles R. Eastman etc. al. 1925-1932.

Chapter- 1:
Origin Of Chordates
Chordata is a group of animals having three important salient features, namely
a dorsal tubular nerve cord, a notochord and gill slits. It comprises about 50,000
species including Balanoglossus, Ascidians, Amphioxus, Petromyzon, fishes,
reptiles, birds and mammals. It is the last and highest phylum among the 30 or
more phyla of the animal kingdom.
Time of Origin
Chordates have originated from invertebrates sometime 500 million years ago
during Cambrian explosion. But which invertebrate stock gave rise to chordates
is difficult to answer because a wide morphological gap exists between the
invertebrates and chordates and there are no significant fossils known to bridge
this gap. The ancestors of chordates were soft bodied and therefore no fossil
records are present, hence the only way of tracing the origin comes from
resemblances between protochordates and invertebrates of the present.
Place of Origin
They may have evolved from some freshwater forms as Chamberlain (1900)
pointed out that all modern chordates possess glomerular kidneys that are
designed to remove excess water from body. However, early fossils of chordates
have all been recovered from marine sediments and even modern protochordates
are all marine forms. Also, glomerular kidneys are also found in some marine
forms such as myxinoids and sharks. That makes the marine origin of chordates
more plausible. So it is believed that the first chordates originated in the sea.
First Chordates

It is believed that sedentary Pterobranchs (Hemichordata) were the first
chordates evolved on the earth and they gave rise to Ascidians. The larvae of
Ascidians evolved into cephalochordates and fish by neoteny.
Probable Ancestor
Though there are several theories to explain the ancestry of chordates, the view
of Barrington (1965) is convincing. According to him, the ancestor of chordates
was a sessile lophophorate or arm feeding creature. It was an invertebrate
dueterostome.
Theories of Origin of Chordata
Several theories to explain the origin of chordates have been given in the past.
All the early theories are far from being satisfactory and have only historical
value. Few modern theories which are convincing are given below:
1. Echinoderm Origin. The theory was given by Johannes Muller (1860) and is
based on the comparative studies of larval stages of echinoderms and
hemichordates. Tornaria larva of hemichordates resembles echinoderm larvae
such as Bipinnaria, Auricularia, Dipleurula and Doliolaria, which all possess
ciliary bands and apical tuft of cilia. Johannes Muller, W. Garstang and DeBeers
proposed that echinoderm larvae gave rise to chordates by neoteny. Also, like
chordates, echinoderms are also deuterostomes and possess mesodermal skeletal
elements.
The discovery of fossil echinoderms called Calcichordata from Ordovician period
(450 mya) further con-firms echinoderm ancestry of chordates. Calcichordates
were asymmetrical animals which demonstrate affinities with both echinoderms
and chordates but their skeleton is made of CaCO3 whereas in vertebrates the
bones are made of hydrated Ca and phosphate. They had large pharynx with a
series of gill slits, each covered with flaps for filter feeding, a small segmented
body and a postanal tail. A perforated pharynx for filter feeding appears to have
evolved in diverse groups of animals during Cambrian-Orodovician periods when
planktons were abundant in water.

2. Hemichordate Origin. Romer (1959) suggested that ancestral
deuterostomes were sedentary tentacle feeders whose mucous-laden ciliated
tentacles served to trap planktons as they were waved in water as do the modern
lophophorates and pterobranch hemichordates, Cephalodiscus and
Rhabdopleura. By some mutation pharyngeal gill slits evolved in these
ancestors, which made the pharynx sieve-like to trap planktons as the water
current passed through it. Extant pterobranchs possess both ciliated arms and
pharyngeal gill slits. Tornaria larva of hemichordates shows phylogenetic
relationship with echinoderm larvae and hemichordates also show affinities with
chordates.
3. Urochordate Origin. W. Garstang (1928) and N.J. Berrill (1955) gave
importance to the tadpole-like larva of urochordates which carries typical
chordate characters, namely, a notochord in tail along with segmented
myotomes, dorsal hollow nerve cord, sense organs and pharyngeal gill slits.
Garstang (1928) suggested that chordates evolved from some sessile filter
feeding urochordate by the larval stage evolving into adult by neoteny and by
losing the sedentary adult stage.
4. Cephalochordate Origin. Chamberlain (1900) studied the primitive and
advanced characters of ceph-alochordates and proposed that while extant
cephalochordates possess all chordate characters in typical state, they also show
some primitive features of non-chordates, such as, absence of heart, head, sense
organs, respiratory pigment, filter-feeding mode of food capture and excretion by
solenocytes. Fossils of 60 specimens from mid-Cambrian of the earliest chordate,
Pikaia gracilens have been discovered from Burgess Shale in British Columbia,
Canada. The Amphioxus-like fossils show streamlined, ribbon-shaped, 5 cm long
body having notochord in the posterior two-third of body and myomeres. It has a
small head with two tentacles and gill slits in the neck region. Other chordate-
like fossils are: Cathaymyrus from early Cambrian sediments in China and
Palaeobranchiostomata from early Permian from South Africa that appears to be
more similar to Amphioxus.

5. Combined theory. E.J.W. Barrington (1965) combined all the above theories
and proposed that the common ancestor of echinoderms and chordates was a
sessile ciliary arm feeder that lived in the plankton-rich environment of the
Cambrian. Modern Crinoidea (Echinodermata), Pogonophora and Pterobranch
hemichordates evolved from a similar ancestor by retaining the original mode of
feeding, perhaps be-cause they continued to inhabit the same environment as
occurred in ancestral days. However, pharyngotremy (perforation of pharynx
with gill slits) must have evolved in a large number of groups at that time, which
must have been much more superior method of food gathering by filtering water
through pharynx as compared to ciliated arm feeding. Hence, the sedentary
Protoascidians of that time lost ciliated arm feeding and adopted pharyngeal
filter feeding as the only method of food gathering. Sometime later, when the
plankton population in water declined, free-swimming tailed larva of these
urochordates did not metamorphose and became a neotenic adult, since free-
swimming mode was superior in food searching at a time of food scarcity.
Cephalochordate-like ancestors evolved by perfection and expansion of chordate
characters that were already present in the ascidian tadpole larva. We already
have fossils of such primitive chordates, e.g. Pikaia gracilens from mid-
Cambrian.

BRANCHIOSTOMA
(The Lancelet)
Systematic Position
Phylum - Chordata
Subphylum - Cephalochordata
Class - Leptocardii
Family - Branchiostomidae
Type - Branchiostoma (Amphioxus)

General account
 Branchiostoma lanceolatus was first discovered by Pallas in 1778. He
considered it as mollusc and named it limax lanceolatus.
 Costa first recognized it as a lower chordate in 1834, and described it as
Branchiostoma.
 The name Amphioxus was given two years later by Yarrell.
Habit and Habitat
 Branchiostoma is a marine chordate. It inhabits the shallow water of the
sandy coasts.
 Branchiostoma is a burrowing animal and remains for most of the time in
its burrow, keeping anterior part of the body protruding out to draw in a
water current.
 At night, it leaves the burrow and swims by lateral undulations of the
body.
 It is a ciliary feeder and feeds on microscopic planktons, brought along
with a respiratory cum food water current which enters the mouth.
 Sexes are separate but alike externally. Fertilization is external.
Development is indirect involving a free-swimming larval stage.
External Characters
 Branchiostoma has a whitish, translucent body, 5 to 8 cm. long, laterally
compressed; both the ends are pointed and lance like, hence the common
name lancelet (a little lance).
 The body is divisible into two regions only: a long anterior trunk and a
short posterior tail. A true head is absent. The anterior end of trunk is
called the rostrum or snout.

 It lacks paired fins, but bears three median or unpaired fins; a dorsal, a
ventral and caudal. The dorsal fin is quite low and extends along the
dorsal border of the entire trunk. The ventral fin is a little wider and run
mid-ventrally from caudal fin up to atriopore. The caudal fin extends
round the tail vertically. It is wider than and continuous with the dorsal
and ventral fin.
 The dorsal and ventral fins are supported by small rectangular fin ray
boxes. The latter are pockets of connective tissue, each containing a
central nodule. There is a single row of such boxes in the dorsal fin, but
two rows (right and left) in the ventral fin. The caudal fin is without fin
ray boxes.
 The trunk has three apertures: mouth, atriopore and anus.
 The mouth is very wide and leads into the oral hood. Its margin is beset
with about eleven pairs of slender but stiff processes, the oral cirri or
buccal tentacles.
 The atriopore is a small mid ventral aperture situated just in front of the
ventral fin. It serves as an outlet for atrium present round the pharynx.
 The anus lies at the base of caudal fin on the ventral side, but a little to
the left side of the median line.
 Ventral surface of anterior two thirds of the trunk is nearly flat and is
called the epipleura. Its lateral margins are produced downwards into a
pair of thin folds, the matapleural folds. These are continuous in front

with the lateral edges of the oral hood and unite together posterior to the
atriopore. These help in borrowing in sand.
Body Wall
 The body wall consists of skin, muscles and peritoneum.
 The skin shows two regions: the outer epidermis and the inner dermis.
 Epidermis. The epidermis is very thin having a single layer of columnar
cells and rest on a basement membrane. They are ciliated in the young
individuals. In the adults, they lose cilia and secrete a thin layer of
iridescent, but non-pigmented, chitin like cuticle externally. The epidermis
contains gland (mucous) cells and sensory cells the cuticle is perforated
over both these types of cells.
 Dermis. The dermis is composed of connective tissue. It is differentiated
into an outer thin, compact, layer crowded with fibres and an inner thick,
spongy layer with fewer fibres, connectives tissue cells, blood vessels and
nerve fibres.
 Muscles. They lie under the skin. In the dorsal and dorsolateral regions,
they are very thick and show metameric segmentation and are arranged
in a linear series of V-shaped muscle blocks, the myotomes or
myomeres. There are about 60 myotomes on each side and the myotomes
of the left side alternate with those of the right side. Each myotome is
enclosed in a complete envelope of connective tissue called the myosepta
or myocommata.
 The muscles of the ventral and ventro lateral regions are thin, un-
segmented and transverse.
 All the body muscles are striated (voluntary).
 The lancelet for the first time in the chordates shows clear cut body
segmentation and the consequent ability to swim by lateral body flexion.

 Peritoneum. The peritoneum covers the muscles internally. It consists of
a layer of thin cells resting on a basement membrane. In the pharyngeal
region the peritoneum is restricted to certain small tracts. The peritoneum
secretes the coelomic fluid.
Skeleton
 There is no exoskeleton in Branchiostoma. Endoskeleton includes the
notochord, dense fibrous connective tissue, gelatinous rods and plates, and
fin ray boxes.
 Notochord. The notochord is in the form of a rod that extends the whole
length of the body along the mid dorsal line above the gut and beneath the
nerve cord. Anteriorly, it reaches ahead of the myotomes and the brain,
quite unlike the pos.
 In early developmental stage, the notochord is made of large vacuolated
cells filled with fluid like secretions. But in adult, it is composed of a linear

series of alternate disc-like fibrous and homogenous gelatinous plates. A
fluid fills the space between these plates.
 Externally, the notochord is surrounded by a laminated tough fibrous
connective tissue notochordal sheath,
 The notochord serves as an axial skeleton, maintaining form of the body
and preventing its shortening on contraction of myotomes. It, however,
permits bending of the body.
 It should be noted that the notochord does not support the myotomes and
other visceral structure like that of vertebral column.
 Dense Fibrous Connective Tissue. This tissue provides firmness to the
body by filling up spaces between its organs. It is continuous throughout
the body and forms a layer inside the epidermis and outside the parietal
layer and forms a layer inside the epidermis and outside the parietal
peritoneum, surrounds the notochord and covers the central nervous
system. It also encloses the myotomes, whose fibres and inserted into it.
 Gelatinous Rods and Plates. They support the gill bars of the pharynx.
Oral cirri free anterior edge of the oral hood. The gelatinous plates support
the floor of the endostyle. The rods and plates are formed of agglutinated
elastic fibres are firm though flexible.
 Fin-ray-Boxes. The fin-ray boxes support the dorsal and ventral fins.
There is a single row of such boxes in the dorsal fin, but two rows (right
and left) in the ventral fin. These boxes are blocks of gelatinous matter
wrapped by connective tissue.
 Besides the above skeletal structure, the fluid-filled coelomic spaces also
serve a skeletal role.
Coelom and Atrium
 Coelom. Branchiostoma, like the vertebrates, possesses a true coelom
lined by peritoneum and filled with coelomic fluid. It is spacious in the

intestinal region, but occurs in its typical state only in a small portion in
front of the anus. Here it uniformly surrounds the intestine except on the
dorsal side, where the dorsal mesentery suspends the intestine into the
coelom.
 In the pharyngeal region the coelom is greatly reduced in the adult stage.
It is represented by a pair of longitudinal dorsal pharyngeal coelomic
canals above the pharynx, a longitudinal midventral coelomic canal or
subendostylar canal beneath the pharynx and a double series of vertical
coelomic canals in the primary gill bars. In the higher chordates coelom is
altogether lost in the pharyngeal region.
 Small closed coelomic spaces also occur around the mid gut diverticulum’s
and in the gonads.
 Atrium. The atrium is an ectoderm lined cavity surrounding the pharynx,
oesophagus and intestine ventrally and laterally and opening out by
atriopore.
 Gill slits of Branchiostoma, unlike those of higher chordates, do not open
directly to the exterior, but into the atrium, which in turn communicates
with the exterior by a small aperture, the atropine, situated on the ventral
side of the trunk just in front of the ventral fin.
 Posteriorly, the atrium extends behind the atriopore as a blind pouch on
the right side of intestine up to anus.
 Anteriorly, the atrium projects into each dorsal coelomic canal on both side
of pharynx forming the Brown funnel or Atrio-coelomic canal of
unknown function.
Movement and Locomotion
 Locomotion in Branchiostoma takes place by swimming that result from
lateral undulations of the body. The muscles that cause lateral
undulations have longitudinal fibres split up into myotomes by myosepta.

 The lancelet burrows with the anterior end foremost. This end, being
stiffened by notochord, acts as a sort of drill. Force to drive the drill
through the sand is provided by the lateral flexures of the body similar to
those that bring about swimming.
 The animal normally lies with most of the body buried and anterior end
protruding. At the time of danger, the anterior end is withdrawn by
simultaneous contraction of all the myotomes.
Digestive System
 Alimentary Canal. The alimentary canal of Branchiostoma is complete,
straight tube of varying diameter and lined throughout by ciliated
epithelium.
 Mouth. The mouth is a wide aperture at the anterior end of the oral hood
overhung by the rostrum. It is fringed with 10 or 11 pairs of slender
processes, the oral cirri or buccal tentacles, bearing sensory papillae. The
buccal cirri and the edge of oral hood are internally supported by stiff,
gelatinous skeletal rods. The oral cirri form a sort of sieve during feeding.
 Buccal Cavity. The space enclosed under the oral hood is the buccal
cavity. It is lined with ectoderm, constituting a sort of stomodaeum. Its
lining is folded to form a number of thick, fingers like, ciliated ridges; each
with a groove along is middle. All the ridges are together referred to as the
wheeler organ or rotatory organ or Muller’s organ because of its shape and
whirling water currents set up by it during life.
 The mid dorsal groove of wheeler organ is the largest which ends in a
small depression on the roof of buccal cavity. These are named
Hatschek’s groove and Hatschek’s pit, respectively. Both are ciliated,
glandular and secret mucus, while the pit is also considered a sensory
organ of unknown function.

 A thin, circular, vertical partition, the velum, bounds the buccal cavity
posteriorly. It has its centre a circular aperture, called the enterostome
that leads into the pharynx.
 The velum is provided with a sphincter to open or close the enterostome.
The posterior border of velum is produced into 10 or 12 velar tentacles,
like that of oral cirri bearing sensory papillae and act as a strainer during
feeding.
 Pharynx. The pharynx is the largest part of the alimentary canal and
occupies nearly half of the body. It is laterally compressed sac with a
complex structure. Its wall is perforated on either side by a row of about
180 narrow, semi vertical (sloping antero-posteriorly) clefts called the gill
slits or branchial apertures. These clefts put the cavity of the pharynx in
communication with the atrial cavity. Number of gill slits increases with
the age of the animal by adding new gill slits to the posterior end of the
series.
 The gill slits are separated from each other by gill bars. The number of gill
slits in the young lancelet is much less, later on, each gill slit becomes sub-
divided into an anterior and a posterior half by vertical growth from the
dorsal wall of the gill slit called secondary or tongue bar. The original
gill slits are, therefore, known as primary gill slits and their separating
bars are called primary gill bars, while the subdivided gill slits are
known as secondary gill slits or stigmata.
 The primary and secondary gill bars are interconnected by horizontal
cross bars called the syn-apticula. A synapticulum contains a skeletal
rod with a blood vessel in continuation with the similar structures of the
gill bars.
 The pharynx has a shallow groove, the endostyle, along the mid ventral
line of its entire length. It is lined by 5 longitudinal tracts of ciliated cells
alternating with 4 tracts of mucus secreting gland cells. The endostyle is

supported by two skeletal plates, beneath which is the subendostylar
coelom containing the ventral aorta.
 It is important to note that similar endostyle occurs in the tunicates
(Herdmania) and in the larva of lamprey. In the larva of the lamprey the
endostyle disappears during metamorphosis, but takes part in the
formation of the thyroid gland of the adult. Further, like the thyroid of the
craniates, the endostyle concentrates iodine in itself and the extract from
the endostyle stimulates the action of the thyroid hormone.
 A pair of peripharyngeal band in the prebranchial area passes upwards
and backwards from the anterior end of the mid-dorsal line, where they
approach each other and proceed to meet a dorsal epipharyngeal or hyper-
pharyngeal groove terminating in the oesophagus.
 Oesophagus. The oesophagus follows the pharynx. It is a short, narrow,
ciliated tube and leads into the intestine.
 Intestine. The intestine is about as long as the pharynx. It shows three
regions: anterior wide midgut, middle short ilio-colic ring, and posterior
tapering hind gut.

 The mid gut has a lateral ciliated tract on its right wall. The cilia of this
tract beat downward towards a groove that starts just within the mid gut
diverticulum. The groove is lined with a tract of cilia that beat forwards.
 The ilio-colonic ring is heavily ciliated and serves to rotates the food
cord on its longitudinal axis.
 The hind gut has a dorsal ciliated groove that starts from the iliocolonic
ring and extends posteriorly. A small terminal part of the hind gut is
heavily ciliated and may be termed the rectum. The latter opens out by
anus.
 Anus. The anus is a small circular aperture at the base of the caudal fin
on the ventral side, but a little to the left of the median line. It is
controlled by a sphincter muscle.
 Digestive Glands. The whole intestine, except the ilio colic ring, has
gland cells scattered in its epithelium. Besides these, the mid gut
diverticulum is the main digestive gland. It arises as a blind pouch from
the ventral junction of oesophagus and mid gut and extends forward
through the atrial cavity along the right side of pharynx. It is surrounded
by a narrow coelomic cavity. Its inner lining has a strong ciliated groove
for movement of food.
Feeding and digestion
 Branchiostoma is ciliary or filter feeder. The animal remain buried in the
sand only the oral hood rising above the sand. The rotatory movements of
cilia of wheeler organ cause a water current into mouth.

 The buccal cirri are folded in front of the mouth to form a fine sieve that
prevents the entry of larger food and sand particle.
 The water current is subjected to further filtering while passing through
the enterostome, where the velar tentacles, like the oral cirri, form a sieve
over the aperture. So that only very fine food particle enters the pharynx.
 The chemoreceptor present over the velar tentacles and buccal cirri
probably taste the water current and the food particles.
 Food particles that escape the main current, are caught up and
concentrated by mucus secreted by the Hatschek’s groove and pit in the
oral hood and pushed through enterostome into pharynx by whirling
action of wheel organ and join the main current.
 Periodically the velar tentacles and oral cirri get rid of the large food
particles and sand, settled on them and obstructing the water current by
forcing out a violent rejection current. The rejection current is produced by
sudden contraction of the transverse muscles of the atrial floor and closer
of atriopore.
 Inside pharynx, the food particles get entrapped in mucus secreted by the
glandular tracts of endostyle and by pharyngeal epithelium. The cilia of
gill bars beat upwards so that mucous sheets laden with food particles
move dorsally to the epipharyngeal groove.
 The food particles settling down in the prebranchial region of pharynx are
also swept up by the cilia of peripharyngeal bands to the mid-dorsal
epipharyngeal groove. The cilia of epipharyngeal groove beat backward
moving the food laden mucus into oesophagus in the form of a narrow food
cord or food cylinder.
 From the oesophagus the food is directed into the cavity of the midgut
diverticulum by lateral patch of cilia on the left wall of mid gut and driven
again to midgut by ciliary action.

 Enzymes are secreted by the midgut diverticulum and are passed on into
the midgut by ciliary action. Similar enzymes are secreted by gland cells of
midgut and hindgut. Digestion is mainly extracellular.
 Upon arriving at the ilio-colic ring, the food cord is thrown into a spiral
coil and rotated by the action of cilia of this ring.
 Broken pieces from the cord pass into the hindgut. Absorption takes place
mainly in the hindgut and digestion in the hindgut is mostly intracellular.
 Chief enzymes found in the midgut diverticulum and the hindgut is
amylase, lipase and protease.
 Phagocytic cells also occur in the renal papillae present on the atrial floor.
These cells engulf small food particles that occasionally escape into the
atrium.

Blood Vascular System
 The blood vascular system of Branchiostoma is well developed and closed.
 It is peculiar in many ways - (i) lacking a heart, (ii) absence of a
respiratory pigment, and (iii) lack of a structural distinction between
arteries, veins and capillaries. However, the principal blood vessels are
homologous with those of higher chordates and are therefore, given the
same names.
 Blood Vessels. The principal blood vessels of Branchiostoma include
ventral aorta, dorsal aorta, subintestinal veins, hepatic portal and hepatic
veins, and cardinal veins.
 Sinus venosus. It is present below the posterior end of pharynx as large
thin walled sac, which receive returning blood from all parts of the body
through veins and pours it into ventral or endostylar aorta.
 Ventral Aorta or endostylar aorta is a median longitudinal vessel that lies
beneath the pharynx in the subendostylar coelom. Blood flows forwards
through it by rhythmical contractions of its muscular wall.
 The ventral aorta gives off paired lateral branches, the afferent branchial
arteries that pass into the primary gill bars. Each afferent branchial
artery has at its base has a contractile tiny bulb, called bulbillus or
bulbule, which assists in the circulation of the blood.
 The afferent branchial arteries of secondary gill bars receive blood from
those of the primary gill bars by means of small transverse arteries
passing through the synapticula.
 The afferent vessels leave the pharynx dorsally as such in the form of
efferent branchial vessels, which open into a lateral dorsal aorta of that
side. Before doing so, each efferent vessel splits into small capillary
network, the nephirc glomerular sinus or glomus, in relation with a
nephridium.

 Dorsal Aorta. The right and left lateral dorsal aortae lie on either dorso-
lateral side of pharynx. They extend forward as the carotid arteries to the
hood region. Behind the pharynx, the two unite to form a single vessel, the
median dorsal aorta which runs posteriorly between the notochord and
intestine and enters the tail region as caudal artery.
 The main flow of the blood in the lateral dorsal aortae is backward.
 Sub-intestinal Vein. The sub intestinal veins lies beneath the intestine.
It has the form of a plexus rather than a single vessel and receives blood
from the intestinal wall. Blood flows forward through it. Posteriorly, the
plexus receives a median caudal vein from the tail.
 Hepatic Portal and Hepatic Veins. Sub-intestinal vein continues
anteriorly as a single wide vessel, the hepatic portal vein, along the
ventral surface of the mid gut diverticulum. It gives off several minute
vessels that from a network in the wall of the diverticulum. The vessels
returning blood from the mid gut diverticulum join to form the hepatic
vein on its upper surface. The hepatic vein opens into sinus venosus,
situated below the posterior end of the pharynx.
 Cardinal Veins. The blood from ventro-lateral region of body is collected
on either side by an anterior and a posterior cardinal vein. Both the veins
unite just behind the pharynx and form the ductus Cuvieri or common
cardinal vein. The ductus Curvieri discharge blood into the venous sinus.

 Parietal veins. A pair of parietal veins run above the intestine and after
collecting blood from dorsal body wall, it turn ventrally and join the sinus
venosus.
 Blood flows anteriorly inside parietal, sub-intestinal and posterior
cardinal veins and ventral aorta.
 Blood flows posteriorly inside lateral and median dorsal aortae and
anterior cardinal vein.
 Blood. Blood of Branchiostoma is colourless and lacks corpuscles. Its
circulation is slow due to infrequent and ill coordinated contractions of the
main vessels. Blood pressure is low. The main function of the blood is
transport of food, and it plays very little part in the transport of oxygen.
Respiratory System
 Branchiostoma lacks special respiratory organs. Exchange of gases takes
place by diffusion through the body surface in contact with the sea water,
namely, epidermis, branchial epithelium and atrial epithelium.
 Pharynx plays a minor role in respiration. This is because the blood lacks
a respiratory pigment to increases its oxygen absorbing capacity and it
passes through the gill bars in direct vessel without spreading out in
capillary network.
 The oxygen absorbed by the epithelium of the gill bars is consumed in
working their cilia so that the blood leaving the gill bars is hardly more
oxygenated than that entering them.
Excretory System
 The excretory organs of the Branchiostoma are ectodermal protonephridia
or segmental vesicles. About 90-100 segmentally arranged protonephridia
are arranged on either side of the pharynx.

 A nephridium is a small, thin walled sac having a long anterior vertical
limb and a short posterior horizontal limb. The vertical limb lies in the
coelomic canal of the primary gill bar and ends blindly.
 The horizontal limb also lies in the dorsal coelomic canal but opens into
the atrium opposite a secondary gill bar by nephridiopore.
 Numerous short branches arise from the sides of the body of nephridium;
each receives a tuft of flame cells or solenocytes. The entire nephridium
carries about 500 solenocytes.
 Each solenocyte is nearly 50μ long and consists of a long hollow stalk or
tubule that opens into the lumen of nephridial branch through a separate
aperture.
 A long flagellum runs through the tubules of the solenocyte to drive the
fluid into the body of the nephridium.
 A single large nephridium is situated above the oral hood on the left side
of the median line. It is known as the Hatschek’s nephridium and
resembles the paired nephridia in all essential respects. It is narrow tube,
which opens at it hind end into the pharynx just behind the velum and
ends blindly just in the front of the Hatschek’s pit.
 Brown funnels and Renal papillae also play some role in excretion.

Nervous System
 Nervous system of Branchiostoma is very simple. It is divisible into the
usual three parts: central, peripheral and autonomic.
 Central Nervous System. It is consists of a dorsal hollow nerve cord or
neural tube lying in the middorsal line just above the notochord. Its
anterior end terminates abruptly in the rostrum, where it shows a slight
enlargement, the cerebral vesicle or the so-called brain.
 The cerebral vesicle contains two important receptor organs, a pigment
spot in its anterior wall and an infundibular organ on its floor.
 The nerve cord encloses throughout its length a narrow cavity, the
neurocoel or central canal. This is full of cerebrospinal fluid. It dilates
within the cerebral vesicle and forms its ventricle. A pouch like blind
dorsal diverticulum arises from its roof and runs behind over the
central canal for a short distance.
 The nerve cord consists of inner grey matter of nerve cells surrounding the
central canal, and outer white matter of nerve fibres, which resembles
with the other vertebrates.
 Peripheral Nervous System. The peripheral nervous system consists of
two pairs of cerebral nerves and a segmental series of paired spinal
nerves.
o (i) Cerebral Nerves. Both pairs of cerebral nerves originate from
the sense organs of the snout, oral hood and oral cirri and carry
sensory impulses to the nerve cord. Hence they are sensory in
nature.
o (ii) Spinal Nerves. Spinal nerves arise from nerve cord behind the
cerebral vesicle; one pair of these arises on either side in each
segment. In each segment there are a pair of dorsal roots and a
pair of ventral roots, but the dorsal and ventral roots of a side do

not unite to form a mixed spinal nerve as happens in higher
chordates.
 A ventral root consists of a group of nerves, terminating in myotome. This
root is purely motor or efferent.
 The dorsal root is both sensory and motor. It is mixed in nature, passes
out to the skin.
 Autonomic Nervous System. The autonomic nervous system controls
the involuntary muscles in the wall of the gut. It consists of two nerve
plexuses in the gut wall. These communicate with the nerve cord by
means of visceral nerves in the dorsal roots.
Sense Organs
 The sense organs of Branchiostoma are very simple, consisting of isolated
sensory cells or small group of such cells.
 Eyes. The eyes, also called eye spots or ocelli, are sensitive to light. An eye
consists of two cells: an outer cup-shaped pigment cell and an inner
photosensitive cell. The eyes are arranged in two ventro-lateral tracts
along the central canal of the nerve cord.
 Cephalic Pigment Spot. It is a very large pigment spot in the anterior
wall of the cerebral vesicle. It lacks the lens and other accessory apparatus
and is not sensitive to light. It serves to shield the photoreceptors from
light from the front. This pigment spot is also thought to be a thermal
sense organ.
 Infundibular Organ. The infundibular organ is a patch of columnar,
ciliated cells in the floor of the cerebral vesicle. The cells of this organ were
earlier thought to detect changes in the pressure of the cerebrospinal fluid
in the neural tube. Later, the infundibular organ was regarded as an eye
stimulated by shadow on it by the cephalic pigment spot. Recently, the
cells of this organ have been found to be secretory in function.

 Kolliker’s Pit. It is ciliated ectodermal depression above the anterior end
of the cerebral vesicle, but slightly to the left side of the median line. It is
sometimes known as the olfactory pit by analogy with the single median
nostril of the cyclostomes. Its epithelium does not have sensory cells. it is
probably a remnant of the neuropore. Some workers consider it a
chemoreceptor.
 Sensory Papillae. The sensory papillae are small groups of sensory cells
and occur on the velar tentacles and oral cirri.
 Sensory Cells. Sensory cells are scattered amongst the epidermal cells
covering the body. They are particularly abundant on the hood. These
sensory cells are tactile in function. Some of these perceive the nature of
sand. The animal avoids too fine sand. Certain sensory cells over the oral
hood are called the cells of Joseph.
 Free Nerve Endings. Free nerve endings are found in the muscles.
These are sensitive to internal changes caused by muscular contractions.
These are called the proprioceptors as compared to the skin receptors,
which are termed the exteroceptors.
Reproductive System
 Sexes are separates in Branchiostoma but there is no sexual dimorphism.
The gonads, testes or ovaries, occur in two rows one on either side of the
body. They show a metameric arrangement, a pair of them in each of the
segments 25 to 51. Their total number is thus, 27 pair. They lie in the
inner surface of the ventro-lateral body atrium.
 Gonads are simple hollow sacs, mesodermal in origin and bulge into the
atrial cavity. They are covered on the outer side by the body wall and on
the inner side by the atrial epithelium.
 Each gonad contains an outer secondary gonadial cavity or gonocoel
around and an inner primary gonadial cavity surrounding a group of germ
cells which arise from its wall.

 Gonoducts are absent. The mature gametes, spermatozoa and ova, are
shed into the atrium by bursting of the inner walls of the gonads at certain
points called cicatrices. From the atrium the gametes escape through the
atriopore into the sea water, where fertilizations and development take
place.
 Fertilization and Development. Fertilization is external in the
surrounding sea water. Development is indirect involving a larval stage.
HERDMANIA
Systematic Position
Phylum - Chordata
Subphylum - Urochordata
Class - Ascidiacea
Subclass - Pleurogona
Order - Stolidobranchia
Family - Pyuridae
Type - Herdmania (sea squirt)
Gen. Account
 Like other members of subphylum Urochordata or Tunicate, Herdmania is
also exclusively marine.
 The genus Herdmania belongs to the class Ascidiacea popularly called
‘ascidians’ or ‘Sea Squirts’.
 It is solitary and sedentary. Found attached to a rocky sea bottom by a
broad base or embedded in sand by massive foot.
 Sometimes it grows on the shells of living gastropods as a commensal.
 Herdmania is a ciliary feeder. It is a ‘microphages’ animal which feeds on
microscopic animals and plants.

 Herdmania is hermaphrodite. Fertilization is external and development is
indirect through tadpole stage.
External Morphology
 It has oblong bag like or potato like body.
 An average adult measures about 9 x 7 x 4 cm while foot measures 3 to 4
cm.
 Due to distribution of blood capillaries in the test it appears pinkish in
colour. It shows scattered bright red patches due to vascular ampullae.
 The body is covered by test and divisible into two parts: body proper and
foot.
 On the free and of the body proper, two short cylindrical projections called
the branchial and atrial siphons or funnels are present.
 The branchial siphon is a little shorter and bears branchial or incurrent
aperture or mouth at its end.
 The atrial siphon is longer and carries the atrial or recurrent or cloacal
aperture at its tip
 Both the apertures are bounded by four lobes, the lips of test. The margin
of each aperture is marked by a bright red line.
 The branchial aperture is wider than the atrial aperture, which allow
better influx of water carrying food, oxygen etc.
 The whole of the body is enclosed by the ‘test or tunic’. The foot is
entirely made up of test.
 The foot is dirty, often having a lot of foreign matter embedded in or
adhere to it.
 Foot acts as anchor for the animal on sandy bottom, it also acts as
balancer to keep the body erect when detached.

 On the hard substratum, the body proper becomes attached by forming a
broad flat or concave base, and the foot is absent.
 The body of animal has a peculiar orientation. Its branchial aperture
marks the anterior side. The opposite side attached to the substratum is,
therefore, the posterior side. The atrial aperture is on dorsal side and the
opposite side partly attached and partly free is the ventral side
 This abnormal orientation of the adult result from the rotatory change in
the larval organization during metamorphosis.
Test or Tunic
 It is a thick, leathery, translucent protective jacket around the body,
which also acts as an accessory respiratory organ and receptor organ.
 It is continuously replaced from inside by the epidermis of mantle which
secrets it.
 It is consists of a clear, gelatinous matrix having wandering cells or
corpuscles, interlacing fibrils, blood vessels and spicules.
 Matrix is made up of a polysaccharide called tunicine, similar to cellulose.
 Corpuscles are mesodermal in origin.
o Large eosinophilous cells
o Small amoeboid cells

o Small eosinophilous cells
o Spherical vacuolated cells
o Granular receptor cells
o Small branched nerve cells
o Squamous epithelial cells.
 Interlacing fibrils run criss-cross all through the matrix. Some are like
smooth muscle cells while some are like nerve fibres.
 Blood vessels form a network system throughout the test. Near the
surface, the branches from oval or pear shaped terminal knobs or
ampullae responsible for red patches visible on the surface of test.
 The ampullae plays role of accessory respiratory organ as well as receptor
organ, being connected to nerve cells.
 Spicules are calcareous and of two types: minute microscleres (40-80 μm
long) and large megascleres (1.5 to 3.5 μm long).

 Megascleres occurs in all part of body except in heart. These are further of
two types: spindle shaped and pipette shaped.
 Microscleres are confined to test only and resemble a paper pin.
 Spicules form the frame work of certain passages for the blood vessels,
serve to attach the body wall with the test and protect the animal from
predators.
Mantle or Body wall
 It lies just beneath the test. It secretes the test and is attached to it only
around the brachial and atrial apertures.
 It is thick, highly muscular and opaque on the antero-dorsal side but thin
transparent and almost without muscles on the postero-ventral side.
 Mantle encloses a large water filled cavity the atrium. It is composed at 3
layers.
1. Outer epidermis: Mode up of single layer of flat, hexagonal cells. At
the branchial and atrial aperture, it in turn and reach up to the base of
the siphons and forms stomodaeum and proctodaeum.
2. Mesenchyme: It develops from the mesoderm. It consists of connective
tissue containing blood sinuses, muscles fibres, nerve fibres and cells.
Muscles fibres are unstriated and arranged in 3 sets:
(a) Annular muscles - surround the siphon.
(b) Longitudinal muscles-start aperture and radiate beneath annular
muscles up to the middle of body on each side.
(c) Branchioatrial muscles - It extend deeper between the two siphons.
 The connective tissue cells of the mesenchyme are chiefly of amoeboid and
vacuolated type.
3. Inner epidermis: It is ectodermic single layer of flat polygonal cells, which
lines the atrial cavity.

Coelom and Atrium
 Due to overdevelopment of atrium or peri-branchial cavity the true coelom
in Herdmania is absent except in certain doubtful derivatives like the
pericardial cavity, gonads etc.
 The space between the pharynx and the mantle, enclosing visceral organs,
is called atrium.
 Atrium is continuous throughout the body except in the anterior and
ventral regions and called the peri-branchial cavity.
 It communicates with the branchial cavity through stigmata in the wall of
pharynx.
 The wide atrial cavity just above the pharynx is known as cloaca into
which open the anus and gonopore.
 The cloaca opens outside through the atrial siphon and atriopore or atrial
aper-ture.
 Sphincter muscles and atrial tentacles are associated with atrial siphon.
Locomotion and Movement
 Adult animal is sessile and movement is visible only during the
contraction of body which squirt out water through atrial and branchial
siphons.

 Three sets of specialised muscles are responsible for this contraction.
o Oral muscles group
o Atrial muscles group
Digestive System
 The alimentary canal of Herdmania is complete and coiled.
 Mouth is present at the tip of the branchial siphon called as branchial
aperture.
 The cavity of branchial siphon is called stomodaeum or buccal cavity.
 Branchial sphincter and branchial tentacles are associated with
biconcavity, which regulate the entry of food.
 Branchial tentacles are chemoreceptor too.
 Pharynx is the largest part of the alimentary canal and occupies the
greater part of body.
 It is divided in to two ports.
o Pre-bronchial zone
o Branchial zone
 Pre-branchial zone is the smaller anterior region having smooth walls.
 Anterior and posterior peripharyngeal bands, enclosing a peripharyngeal
groove, separate pre branchial zone from branchial zone.
 In front of anterior peripharyngeal band a swollen dorsal tubercle present
mid-dorsally.
 Branchial sac or branchial basket is the larger posterior region of pharynx.
 Its lateral walls are perforated by numerous elongated gill-slits of
stigmata which communicate with atrial cavity.
 The inner wall of branchial sac forms 9 to 10 branchial folds on each side
due to longitudinal folding in wall.

 The outer wall is connected with mantle by several hollow strands called
trabeculae.
 From the roof of the branchial sac a 1 to 1.5 cm long fold is suspended,
called hyper-pharyngeal band or dorsal lamina. It helps in conduction of
food.
 On the floor of the branchial sac, a shallow longitudinal mid-ventral
groove is present called endostyle. Cells of endostyle secrete mucus which
helps in feeding process.
 The endostyle of urochordates is homologous with the hypo-pharyngeal
groove of cephalochordates and thyroid glands of vertebrate.

 Oesophagus is very short, curved and thick walled tube opens in to
stomach.
 Stomach is thin walled, have sphincters at both ends and surrounded by
right and left liver lobes.
 Intestine is thin-walled, proximal descending limb and a distal ascending
limb make it U shaped and which encloses the left gonad.
 Rectum is a short narrow tube, lined by cilia which is open into the atrium
or cloaca though anus, which is bounded by four lips.
 Cloaca leads into atrial siphon which opens to the outside through the
atrial aperture.
Digestive Glands
 Liver is dark, brown and bilobed with a larger left lobe and smaller right
lobe. It open in to stomach and pour digestive secretions.
 Pyloric glands are present in the wall of stomach and intestine; they open
by a single duct in to proximal limb of intestine.
 In Herdmania pyloric gland performs a dual function, that of a vertebrates
pancreas and of an excretory organ.
Food, Feeding and Digestion
 Herdmania is a filter feeder and ciliary feeder.
 Constant water current laden with food enter through stigmata into the
atrial cavity and leaves body through atrial siphon and atrial aperture.
 The branchial tentacles act as chemoreceptors and keep the impurities
out. The larger particles are expelled out of the mouth by a strong reverse
current.
 In stomach secretions of liver digest the food. Secretions of pyloric gland
complete the digestion in intestine and absorption starts.

 The undigested food passes into rectum and further into cloaca though
anus and expelled out through atrial aperture.
 Starch-like granules are present in the liver and walls of alimentary canal
in the form of reserve food.
Respiratory system
 Branchial sac is the main respiratory organ in Herdmania. The wall of
this sac is highly vascular and very thin enabling gaseous exchange.
 Longitudinal folds on the inner surface of the branchial sac further
increases the respiratory surface enormously.
 Exchange of gases also takes place in the trabeculae, which stretch
between the pharyngeal wall and the body wall, which constantly bathed
in fresh sea water leaving the pharynx via atrium.
 Outer surface of test acts as an accessory respiratory organ, where the
vascular ampullae play

Blood vascular system
 The blood vascular system of Herdmania is peculiar in:
o Single chambered valve less heart,
o Periodic reversal of blood flow,
o Lack of capillaries,
o Presence of numerous sinuses, and
o Unusual variety of blood corpuscles.
 It includes: (i) heart and pericardium, (ii) blood vessels and (iii) blood.
 The pericardium has a relatively thick, noncontractile wall composed of
connective tissue containing blood sinuses and lined by Squamous
epithelium. It is closed at both end and filled with a colourless pericardial
fluid with corpuscles similar to those of blood.
 Pericardium is embedded in the mantle on the right side of the body below
the right gonad.
 Heart is a cylindrical highly contractile and thin walled structure with
striated muscles. It is formed by an infolding of the pericardium.
 Both ends of heart are open. There are no valves but a pear shaped body
present which influence the flow of blood in the heart.
 Herdmania has well developed blood vessels. The larger vessels have a
lining of endothelium, whereas the smaller ones are mere spaces in the
connective tissue.
 There are four main blood vessels: i. Ventral aorta, ii. Dorsal aorta, iii.
Branchio-visceral vessel, and iv. Cardio-visceral vessel
 The ventral aorta is the largest vessel, starting from ventral end of heart
and divides into anterior and posterior limbs, which extend forwards and
backwards through the ventral wall of branchial sac just beneath the
endostyle.

 Both this branches further give off several paired transverse branchial
vessels to the wall of branchial sac. These vessels are connected with
longitudinal branchial vessels.
 The anterior limbs become narrow and join two circular vessels, a peri-
pharyngeal vessel and a sub-tentacular vessel.
 The posterior limb proceeds to the oesophageal area as a thin vessels.
 Dorsal aorta runs in the dorsal wall of the brachial sac just above the
dorsal lamina.
 It is not connected to the heart but communicates with the ventral aorta
through transverse vessels of the branchial sac and the peri-pharyngeal
and sub-tentacular vessels.
 Anteriorly, it sends a neural vessel to the neural complex, and then joins
the peri-pharyngeal and sub-tentacular vessels. Posteriorly, it joins the
branchio-visceral vessel.
 Branchio-visceral vessel is a very short vessel. It ends behind in two
branches: (i) right oesophageal vessels that carry blood to the right liver
lobe, right lip of the oesophageal area and the oesophagus, and (ii) ventro-
intestinal vessel that run along the ventral wall of intestine and supplies
blood to the left lip of oesophageal area, left liver lobe, stomach and
intestine.
 The cardio-visceral vessel originated from the dorsal end of the heart. It is
very short vessel. It soon gives off two small branches: the right hepatic
vessel that enters the right lobe of the liver, and the oesophageo-test
vessel that supplies the oesophagus and the test vessels.
 The cardio-visceral vessels then curves to the left side of the body and
divide into three vessels: median dorsal vessel, left gonadial vessel and
gastro-intestinal test vessel.

 Blood is slightly reddish, almost transparent and a little hypertonic to sea
water. It contains eight types of corpuscles: i. orange cells, ii. signet cells,
iii. green cells, iv. compartment cells, v. eosinophilous cells,
vi. lymphocytes, vii. macrophages; and viii. nephrocytes.
 Tunicate blood has a unique property of extracting the element vanadium
from sea water as in Ciona, but Herdmania has lost the power of
extracting vanadium. The function of vanadium is still unknown.
 There is no valve present in the heart of Herdmania and the flow of heart
is maintained by peristaltic waves and the small pear shaped body.
 The ascidians heart has unique property of changing direction of blood
flow by reverse peristalsis at regular intervals.
 When the heart beats ventro-dorsally, its oxygenated blood, collected
through ventral aorta from branchial sac and the test, is pumped into the
cardio-visceral vessel and distributed to the various part of the body (test
and viscera).
 The deoxygenated blood from viscera is collected by the Branchio-visceral
vessel which passes it to the dorsal aorta from where it goes into the
transverse branchial vessels to become oxygenated once again to undergo
fresh cycle.
 When the heart beat is reversed in dorso-ventral direction, the
deoxygenated blood collected through cardio-visceral vessel from viscera,
is pumped into ventral aorta and distributed into transverse branchial,
peri-pharyngeal, sub-tentacular and test vessels.
 The blood now oxygenated, is collected by dorsal aorta and distributed
once again to viscera through Branchio-visceral vessel. Deoxygenated
blood from viscera is collected by cardio-visceral vessel and brought back
to the heart to restart the cycle.

Excretory system
 Neural gland, which lies mid-dorsally embedded in the mantle just above
the nerve ganglion of the brain, is excretory organ of Herdmania.

 The neural gland has a few central tubes, which gives off a large number
of branching peripheral tubules and open into a large longitudinal canal.
 This canal runs along the entire length of the gland and leads into the
neural gland duct. Neural gland duct is open by a ciliated funnel in the
pre-branchial zone at the base of dorsal tubercle.
 Nephrocytes of blood act as execratory cells and collect waste product,
mainly xanthine and ur-ate particles, pass through the lumen of neural
gland and its duct and discharge into the prebranchial zone of pharynx.
 The neural gland has been regarded as an absorptive or sensory or
secretary organ secreting hormones that control oviposition, development
and metamorphosis, and is considered homologous to the vertebrate
pituitary gland.
Nervous system
 It has simple and degenerated nervous system. Still it may be divided into
central and peripheral parts.
 The central nervous system consists of a single, solid, elongated nerve
ganglion, referred as brain. It controls body reflexes.
 The peripheral nervous system consists of three anterior nerves and two
posterior nerves given off from the nerve ganglion.
 The nerve ganglion, neural gland and dorsal tubercle are together referred
as the neural complex.

Sense Organ
 All receptors, except the dorsal tubercle, are very simple in structure,
consisting merely of isolated cells or cell aggregates, with nerve endings.
 Tango receptor cells are scattered in the non-vascular parts of the test,
and the epithelium covering the vascular ampullae and the tentacles.
 Photoreceptor cells are pigmented cells containing red pigment granules,
located on the margins of siphons and vascular ampullae.
 Rheoreceptor cells occur in the rim of the branchial and atrial apertures.
 Chemoreceptor cells are present in the dorsal tubercle and the tentacles.
 Dorsal tubercle serves to smell and taste the water entering the pharynx,
thus functions as an olfactory cum gustatory receptors.

 Tentacles test the quality of incoming water and size of food particles
entering the pharynx. It is regarded as olfactoreceptors.
Reproductive system
 Herdmania is hermaphrodite but protogynous.
 Gonads are two large and embedded in the mantle and cause bulge into
the peri-branchial cavity. The right gonad is situated just parallel and
dorsal to pericardium, while the left gonad lies within the intestinal loop.
 Each gonad consists of 10-15 distinct lobes arranged in two rows with one
median lobe at the proximal end. The median lobe is largest and bean
shaped. Others are ovoid or rounded, and become smaller towards the
distal end of the gonad.
 Each lobe is bisexual, and consists of an outer large and brick red
testicular and an inner small and pink ovarian part.
 Testicular part contains numerous spermatic caeca. The wall of each
caecum consists of a layer of spermatogonia with large nuclei and
surrounds the spermatocytes that give rise to sperms. Mature sperms
become free in the lumen of the caecum.
 The ovarian part has a lobulated surface. It contains rounded ova in
various stages of development.

 Each gonad has two gonoducts oviduct and spermatic duct, running along
the central axis. Both are lined by cilia internally.
 The oviduct is wider and opens into cloaca by an oviductal aperture. The
spermatic duct or vas deferens is narrow duct form by union of spermatic
ductules and open independently into cloaca by a spermiducal aperture.
 The sperms are polymorphic with at least three types having acrosome
shorter, equal and longer than head.
 Ovum is surrounded by three membranes; (i) Vitelline membrane, (ii)
Inner chorion, and (iii) Outer chorion.
 The ovum lies eccentrically in the peri-vitelline fluid enclosed by the space
between the vitelline membrane and inner chorion.
 When the gametes become mature, they are expelled out in sea water
through atrial current. External fertilization takes place.
 Cleavage is holobastic, unequal and determinate.

Chapter – 2: Protozoa
LOCOMOTION IN PROTOZOA
Protozoa possess highly variable locomotory organs, which is also the basis for
the classification of Protozoa.
Locomotory organelles
There are four types of locomotory organelles found in protozoa. These include-
A. Pseudopodia
B. Flagella
C. Cilia
D. Pellicular contractile structures
PSEUDOPODIA
Structure of Pseudopodia- Pseudopodia, also known as false feet, are
temporary structures formed by the streaming flow of cytoplasm. They are of
four types:
1. Lobopodia: - Lobe-like blunt pseudopodia composed of both ectoplasm
and endoplasm, e.g. Amoeba.
2. Filopodia: - Filamentous of thread like pseudopodia composed of
ectoplasm only, e.g. Euglypha.

3. Reticulopodia:- Branched and interconnected filamentous
pseudopodia that display two-way flow of cytoplasm, e.g. Globigerina.
4. Axopodia:- Straight pseudopodia radiating from the surface of the body
and internally supported by an axial thread. They display two-way flow
of cytoplasm, e.g. Actinophrys.
Method of locomotion by pseudopodia (Amoeboid movement)- It is
characteristic of all Sarcodines and certain Mastigophora and Sporozoa. The first
observation of amoeboid movement was noticed by Rosel von Resenhof in 1755.
Since then several theories have been proposed, out of which, sol-gel theory put
forward by Hyman (1917) and later supported by Pantin (1923-26) and Mast
(1925) is the most widely accepted. It attributes amoeboid motion to change in
the consistency of cytoplasm. Based on the spontaneous sol-gel phenomenon of
protoplasm, in which according to need sol can change into gel and vice versa. it
offers the best explanation for amoeboid locomotion. According to the sol-gel or
change of viscosity theory, cytoplasm of amoeba is differentiated into a clear
outer ectoplasm and a granular inner endoplasm. The latter is further
distinguished into an outer stiffer and jelly-like region, the plasmagel and an
inner fluid region, the plasmasol. Amoeboid movement involves four processes
that occurs simultaneously –

(1) The outermost thin, elastic cell membrane or plasmalemma becomes
attached to the substratum.
(2) There is a local partial liquefaction of the plasmagel at the anterior end.
This causes the central plasmasol, under tension, to flow forward and
force the plasmagel against this weakened area to produce a bulge or
pseudopodium. As plasmasol enters the newly formed pseudopodium, it
rapidly changes into plasmagel around the periphery (gelation), thus
forming a gelatinized tube within which the plasmagel continues to flow
forward.
(3) Posteriorly, inner surface of contractile plasmagel undergoes solation, so
that a constant flow of plasmasol is maintained from behind forward in
the direction of movement.
(4) The outer tube of elastic plasmagel contracts and moves from in front
backwards, while the main bulk of body travels forward. The plasmagel
thus exerts a squeezing motion from the sides and rear of amoeba, forcing
the plasmasol to flow forward. At the tip of pseudopodium the endoplasm
is changed to ectoplasm.
FLAGELLA
Structure of flagella- Flagella are thread like
projections on the cell surface of flagellate
protozoa like Euglena, Trypanosoma, etc. A
typical flagellum consists of an elongated, stiff
axial filament, the axoneme, enclosed by an
outer sheath. The axoneme consists of nine
outer double microtubules that encircle two
central single microtubules, forming the typical
9 x 2 + 2 pattern seen in cross-sections. Each of
the peripheral pairs bears a double row of short
arms (containing the motor molecule dynein).
Axoneme arises from a basal body (the blepharoplast or kinetosome) that lies

immediately below the cell membrane. Basal bodies resemble an axoneme except
that the outer nine microtubules are triplets and the central singlets are absent
(9 x 3 + 0). The microtubules of each triplet are continuous with an axonemal
doublet. Dynein arms are absent on the basal body triplets. A basal body is
usually anchored in the cell, often to the nucleus and cell membrane, by one or
more cytoskeletal root structures. Some proteinaceous rootlet fibers are
contractile and can, on contraction, pull the flagellum into a shallow pocket or
alter its orientation. When basal bodies are distributed to daughter cells during
mitosis, they typically arrange themselves at each pole of the mitotic spindle and
are then designated as centrioles.
Flagellar movement- It is characteristic of Mastigophora which bears one or
more flagella. The mechanism producing flagellar beat is not exactly known. It is
believed that some or all of the axonemal fibres are involved. According to the
latest sliding tubule theory of flagellar movement, adjacent doublets slide past
each other, causing the entire flagellum to bend. Cross bridges are formed and
energy utilized for the process is supplied by ATP. The flagella need liquid
medium for movement or locomotion. There are three types of flagellar
movements:
(1) Paddle stroke- This is the common movement of a flagellum by which the
animal moves forward, gyrates and is also caused to rotate on its
longitudinal axis. It consists of an effective down stroke with flagellum

held out rigidly, and a relaxed recovery stroke in which flagellum, strongly
curved, is brought forward again.
(2) Undulating motion- Wave-like undulations in flagellum proceeds from tip
to base, pulling the animal forward. Backward movement is caused when
undulations pass from base to tip. When such undulations are spiral, they
cause the organism to rotate in opposite direction.
(3) Simple conical gyration- is the spiral turning of flagellum like a screw.
This exerts propelling action, pulling the animal forward through water
with a spiral rotation as well as gyration (revolving in circles) around the
axis of movement.
CILIA
Structure of cilia- Cilia are short, highly vibratile, small ectoplasmic processes
having oar-like motion. They resemble flagella in their basic structure. Electron
microscope reveals the presence of an external membranous sheath, continuous
with plasma membrane of cell surface and enclosing the fluid matrix. Running
along the entire length of body of cilium are nine paired peripheral fibres and
two central fibres (9 x 2 + 2), all embedded in a matrix. Central fibres are
enclosed within a delicate sheath. In between the outer and inner fibre rings are
present nine spoke-like radial lamellae. In addition to these, one sub-fibre or
microfiber of each peripheral pair bears a double row of short projections called
arms, all pointing in the same direction. Each cilium arises from a thickened
structure, called the basal body or blepharoplast. Basal body shows nine
peripheral subfibril triplets (9 x 3 + 0), each disposed in a twist-like fashion.

Ciliary movement- Mechanism of ciliary movement in ciliates is little studied.
It is now known that cilia are moved in a coordinating system. They move by the
contraction of peripheral fibres located within them. The basal bodies of cilia are
connected to one another by contractile bundles of fibres called kinetodesmata in
such a way that five cilia form one contractile unit called kinety. Successive
contractions of kineties produce a metachronal wave of movement of cilia giving
forward thrust to the animal. The energy needed for fibrillar contraction is
supplied by ATP. Cilia also need liquid medium for their movements. Two types
of movements are seen in ciliates-

(1) Ciliary beats- During the effective stroke, the cilium is outstretched stiffly
and moves in an oar-like fashion, perpendicular to the cell surface. In the
recovery stroke, the cilium flexes and snakes forward parallel to the cell
surface. As the organism moves through the medium, the ciliary beat is
coordinated over the surface of the cell. The cilia in any cross row are all in
the same stage of the beat cycle, while those in front are in an earlier
stage and those behind are in a later stage. This phase shift is seen as
waves, called metachronal waves that pass over the surface of the cell like
wind passes in waves over a wheat field.
(2) Swimming- Large ciliates are the swiftest swimmers. During the mode of
swimming, the animal does not follow a straight tract but rotates spirally
like a rifle bullet along a left- handed helix. The reason for this is two-fold.
Firstly, the body cilia do not beat directly backwards but somewhat
obliquely towards right, so that the animal rotates over to the left on its
long axis. Secondly, the cilia of oral groove strike obliquely and more
vigorously so as to turn the anterior end continually away from the oral
side and move in circles. The combined effect causes the movement of
animal along a fairly straight path, rotating about its axis in an
anticlockwise direction.
PELLICULAR CONTRACTILE STRUCTURES
In many protozoa are found contractile structures in pellicle or ectoplasm called
myonemes. These are present in the form of ridges and grooves (e.g. Euglena),
contractile myofibrils (large ciliates) or microtubules (e.g. trypanosoma). Such
organisms show gliding or wriggling or peristaltic movement, which is also
referred to as gregarine movement.

Nutrition in protozoa
The protozoa display a range of nutritional types, from the entirely plant-like
photosynthetic (or autotrophic) nutrition to the totally animal-like (or
heterotrophic) nutrition, in which bacteria, algae, other protozoa and small
animals like the crustacean copepods constitute the food source. Protozoa also
lead parasitic life, usually doing no or little harm to their hosts, but occasionally
causing serious diseases. Six types of nutrition seen in protozoa are-
1. Holophytic nutrition: All those
phytoflagellates possessing chloroplasts
or chromatophores synthesize their food
by photosynthesis. As energy is supplied
by sunlight to carry on food making
activity, this method involving self-
feeding is also referred to as autotrophic
or phototrophic nutrition. Carbon dioxide
and water acting as raw materials enter
into a complex cycle of chemical reactions
and produce dextrose sugar.
2. Holozoic nutrition: Majority of free-
living protozoa derive nourishment by
ingesting other organisms, both animals
and plants. Such protozoa are called
holozoic and mode of nutrition is said to be
holozoic nutrition. This mode of nutrition
involves development of organelles for food
capture, ingestion, digestion and egestion
of undigested residues. Food of holozoic
protozoa consists of microorganisms like
other protozoans, bacteria, diatoms,

rotifers, crustacean larvae etc. The method involves ingestion of these
organisms which is referred to as phagotrophy or phagocytosis, by which
larger particles, such as bacteria and protozoans are taken up in large
vesicles called food vacuoles. Food in protozoa is digested within food
vacuoles, which usually keep on circulating in the endoplasm. Once food
enters the cell, lysosomes fuse with the endocytic vesicles or food vacuoles.
Lysosomes are membrane-bound organelles that originate from Golgi
bodies and contain acids and hydrolytic enzymes. Release of these
biomolecules into the food vacuole initiates digestion. Eventually, the
products of intracellular digestion diffuse across the vacuole membrane
into the cytoplasm of the cell, where they may be used in metabolism or
stored after synthesis into glycogen and lipids. Indigestible residue of food
is expelled from the cell to the exterior by fusion of the residual vacuole
with the cell membrane in a process called exocytosis.
3. Pinocytosis: In addition to phagocytosis, pinocytosis or cell drinking is
seen in amoeba and certain flagellates and ciliates. This involves ingestion
of liquid food by invagination through the surface of body. Pinocytotic
channels are formed at some parts of body surface to enclose the fluid food
from the surrounding medium. Lower ends of channels are pinched off as
food vacuoles which circulate into the endoplasm. Pinocytosis is induced
only by certain active substances in the medium surrounding the cell, such
as some proteins and many salts. Its physiological significance seems to be
the absorption of high molecular compounds from the external medium.
4. Saprozoic nutrition: It involves absorption of food by osmosis, through
general surface of the body. This method of absorption is also known as
osmotrophy. Food consists of solution of dead organic matter, rendered so
by the decomposing bacteria. This mode of nutrition is found in
Mastigamoeba, and some colorless flagellates (e.g. Chilomonas, Polytoma).
Dissolved food materials, upon which the saprozoic protozoans subsist, are
proteins and carbohydrates.

5. Myxotrophic nutrition: This is a combination of more than one mode of
nutrition. Many protozoa using photosynthesis as a means of food
synthesis also take in some part of their diet in dissolved form by
osmotrophy or solid form by phagotrophy. Flagellates like Euglena and
Peranema nourish themselves by this method.
6. Nutrition of parasites: Parasitic protozoa feed in a variety of ways. The
food getting mechanisms used by parasitic protozoa are generally the
same as those of their non-parasitic relatives.
 Many intestine-inhabiting Zoomastigophora (Trichomonas) have a
distinct mouth or cytostome through which food particles are ingested
by phagotrophy. Many parasitic ciliates, like Nyctotherus and
Balantidinum do the same. Parasitic Sarcodina of the genus
Entamoeba feed by phagotrophy at least at certain stage of their life
cycle.
 Zooflagellates inhabiting blood (e.g. Trypansoma) feed by osmotrophy.
Osmotrophic forms may be either coelozoic or histozoic. Opalina, which
is found in the rectum of frog is coelozoic and absorbs all its food
through the cell surface. The young trophozoite of Monocystis is
histozoic within the sperm morula and it feeds upon the fluid by
osmotrophy.
 Parasitic saprozoic forms may also use directly the serum of their host
blood.
 Many live in the nutrient-rich medium of the body fluids—e.g. the
blood or cells of their host. There they take in energy-rich fluids by
pinocytosis, in which small amounts of the medium are pinched off into
digestive vacuoles either at a specific site, such as the cytostome in
ciliates or the flagellar pocket in trypanosomes or along the surface of
the cell in amoebas.

Reproduction in Protozoa
Reproduction occurs in all protozoans, in some at frequent interval with only a
short period of growth whereas in others, at comparatively longer intervals with
expanded period of growth which may last from days to week. Reproduction in
protozoa is either asexual or sexual.
Asexual Reproduction
In this type, division of parent body occurs to produce one or more young
individuals. It always involves a single parent and neither meiosis nor
fertilization occurs. Nearly all protozoans reproduce by this method. It takes
place by following methods:
1. Binary Fission
2. Plasmotomy
3. Budding
4. Multiple Fission
5. Plasmogamy
6. Regeneration
Sexual Reproduction
In this type, meiotic nuclear division is followed by the union of gametes. The
gametes may arise from different parents (amphimixis) or may come from same
parent (automixis). It helps in the replacement of old nucleus by genetic
recombination for restoration of vigor. It is of the following types:
1. Syngamy
2. Conjugation
3. Parthenogenesis

ASEXUAL REPRODUCTION
This type of reproduction by mitosis occurs in
most protozoa and is the only known mode in
some species.
Binary Fission: This involves the division of one
individual into two approximately equal parts.
The division is not a mere fragmentation but a
complicated process of mitosis, during which
nuclear division or karyokinesis is always
followed by the division of cytoplasm or cytokinesis. The two daughter organisms
produced as a result of binary fission carry all the cytoplasmic organelles of the
parent individual. Division or fission may be either in a transverse plane (e.g.
Paramecium) or Longitudinal plane (e.g. Euglena) or Oblique plane (e.g.
Ceratium) or any plane (e.g. Amoeba)
In shelled Sarcodina (e.g. Euglypha, Arcella) a mass of protoplasm extends from
the opening of shell, which secretes a new shell. This double-shelled organism
now divides into two.
Plasmotomy: This involves the division of multinucleate Protozoa into two or
more smaller multinucleate daughter individuals. It takes place in Pelomyxa,
Opalina and some other forms.

Budding: Budding is the formation of one or more small individuals by
separating from the parent body. Each bud receives a part of the parent nucleus
and later on turns into an adult. The bud is smaller than the parent. When a
parent body produces only one bud it is monotonic (e.g. Vorticella), while in
multiple budding several buds are formed simultaneously (e.g. Ephelota).
Multiple fission: During multiple fissions, nuclear
division is not immediately followed by the division
of the cytoplasm. At first, nucleus divides by a
series of divisions either by repeated binary fission
(e.g. Plasmodium) or by simultaneous multiple
fission (e.g. Aggregata). The body thus becomes
multinucleate. It is quite a common phenomenon
seen in Foraminifera, Radiolaria, Sporozoa and
certain Mastigophora. The process receives
different names according to the particular period in life cycle it occurs-
Schizogony
 Occurs in the life cycle of Plasmodium.
 The resulting individuals are called merozoites.
Sporogony
 Seen in Plasmodium.
 Takes place after sexual reproduction and the products are termed
sporozoites.

Plasmogamy: In certain Rhizopoda and Mycetozoa two or more individuals may
fuse to form a plasmodium in which the nuclei remain distinct and they separate
again unchanged afterwards. The process, which is thus non-sexual and not
syngamy is called plasmogamy.
Regeneration: It has been observed that nucleated pieces of sufficient size may
reform proportional missing parts and may assume normal shape. In ciliates like
Stentor and Euplotes a piece of macronucleus is necessary for regeneration.
Parasitic protozoa usually have slight regeneration capacity.
SEXUAL REPRODUCTION
This type of reproduction is widespread but not universal in protozoans. Many
protozoans undergo sexual activities at irregular intervals but in many cases the
life cycle cannot be completed without syngamy and gametogenesis.
Syngamy: This is the complete fusion of two sex cells or gametes, resulting in
the formation of zygote. The fusion nucleus of zygote is called Synkaryon.
Depending upon the degree of differentiation displayed by the fusing gametes,
Syngamy is of following types.
 Hologamy- Two ordinary mature protozoan individuals themselves
behave as gametes and fuse together to form zygote. E.g. Sarcodina and
Mastigophora (e.g. Copromonas)
 Isogamy- Two fusing gametes are similar in size and shape. E.g.
Monocystis and Chlamydomonas
 Anisogamy- Two fusing gametes are unequal in shape and size. Small or
motile gametes are male or microgametes and large non-motile ones are
the female or macrogametes. E.g. Plasmodium and Volvox.
 Autogamy- Fusion of gametes derived from the same parent cell. E.g.
Actinophrys and Actinosphaerium.
Conjugation (amphimixis): It is the temporary union of two protozoan
individuals (called conjugants) of the same species for the exchange of nuclear
material. It is characteristic of Suctoria and Holotrichia ciliates. Conjugation can

take place between two individuals of the same syngen but belonging to opposite
mating types. The unique feature of conjugation is an exchange of hereditary
material so that each conjugant benefits from a renewed hereditary constitution.
Sonneborn has recognized different syngens in a species of Paramecium and
each syngen includes two mating types. The process of conjugation in
Paramecium is represented as follows:
1. Two individuals or conjugants come in contact and unite.
2. Degeneration of macronucleus and meiotic division of micronucleus.
3. Four haploid daughter micronuclei are produced in each conjugate.
4. Three daughter micronuclei degenerate or become pycnotic and disappear in
each conjugate.
5. The remaining one divides by mitosis forming 2 unequal gametic nuclei - (a)
active migratory gametic nucleus and (b) passive stationary gametic nucleus.
6. Fusion of migratory nucleus of one conjugant with the stationary nucleus of
other conjugant forming a
zygotic nucleus or synkaryon
(process is termed as
amphimixis).
7. Separation of two
conjugating paramecia – now
termed as exconjugants.
8, 9 & 10. In each exconjugant,
zygotic nucleus divides
mitotically three times to
produce eight daughter nuclei.
11. Four daughter nuclei (in each exconjugant) enlarge to become macronuclei
and other 4 become micronuclei. Three micronuclei disintegrate and disappear.
12. Remaining one micronucleus of exconjugant divides with binary fission.

13. Each exconjugant produces 2 daughter paramecia, each containing 2
macronuclei and 1 micronucleus.
14. Further division of each daughter paramecium forms 2 individuals, each
containing one macronucleus and one micronucleus. Thus, each conjugant
produces four daughter individuals at the end of conjugation.
Parthenogenesis: In Actinophrys the gametes which fail to cross-fertilize
develop parthenogenetically. It also occurs in Chlamydomonas and others when
syngamy has been missed. Individuals of Polytoma, which are potential gametes
can grow and divide parthenogenetically.
Encystment:
Encystment is characteristic of the life cycle of many protozoa, including the
majority of freshwater species. In forming a cyst, the protozoan secretes a
thickened envelop about it and becomes inactive. Depending on the species, the
protective cyst is resistant to desiccation or low temperatures and encystment
enables the cell to pass through unfavorable environmental conditions. However,
the more complex life cycles are often characterized by encysted zygotes or by
formation of special reproductive cysts in which fission, gametogenesis, or other
reproductive processes take place. Protozoa may be dispersed over long distances
in either the active or encysted stages. Water currents, wind, and mud and
debris on the bodies of waterbirds and other animals are common means of
dispersal.
Evolution of Sex in Protozoa
Asexual reproduction is generally quick, has less energy demands and is a
simple process and hence the early protozoan adopted this method as the
primary means of multiplication that enabled them to build up large populations
in shortest possible time. Binary fission, multiple fission, budding, plasmotomy
etc. are all asexual methods that are best suited to these animals which
possessed delicate bodies and no apparent means of defence and therefore
needed a speedy way of multiplication. Unlike in sexual reproduction, where only

females reproduce, in asexual method all individuals produce offspring without
the help of other individuals.
ASEXUAL VERSUS SEXUAL REPRODUCTION
If asexual reproduction is simple and efficient, then why animals had to invent
sexual method which is a complex and difficult process? Asexual reproduction
apparently has the following disadvantages:
 Animals reproducing by asexual means are pure races or clones and have
the same genotype. There is no genetic diversity in such populations and
hence natural selection has nothing to choose from. In adverse conditions
or in changing environmental conditions, entire populations can be wiped
out leading to the extinction of species.
 Mutations are immediately expressed in asexual animals as there is no
dominant gene to mask the effect of a new mutant allele. As the majority
of mutations are deleterious and often lethal, such mutations can result in
the death of individuals.
 Deleterious mutations are found to accumulate in asexually reproducing
species, a phenomenon termed as Muller’s Ratchet. Offspring always
have more mutations than their parents and as new mutations are added
generation after generation, deleterious mutations keep on accumulating
in the population.
Sexual reproduction involves creation of two types of individuals producing two
types of gametes whose primary aim is to exchange the genetic material between
individuals and to bring about genetic diversity in the population. The power of
genetic recombination is so great that chances of two individuals being identical
are almost zero. In the fiercely competitive world such a trait is advantageous
and is favoured by natural selection. Sexually reproducing individuals are known
to adapt quickly to the changing environment and are better competitors. Thus,
the genetic recombination enhances the speed of evolution.

Sexual individuals can get rid of the harmful effects of deleterious mutations by
simply mating with a healthy one. Dominant genes of the healthy individual will
suppress the mutant deleterious alleles of the offspring. Recombination can also
bring together favourable alleles when required for fixing and can at the same
time eliminate deleterious ones by bringing them together in homozygous
expression. Therefore, unlike in asexuals, frequent deleterious mutations cannot
harm the sexually reproducing individuals. Sexually reproducing populations
carry a lot of variety for exigencies.
Sexual reproduction, by bringing together a variety of genes increases the speed
of evolution.
Sexual reproduction also provides an opportunity of DNA repair which is so
common in harsh environment. Since homologous chromosomes possess two
copies of DNA, a template is always available to the enzymes to repair the
damage, while in asexuals any damage to DNA is permanent. Problems created
by genetic changes can only be solved by sexual reproduction.
Owing to overwhelming advantages in the struggle for life, an attempt to evolve
sexual reproduction commenced in eukaryotes. A nucleus and the cell division
apparatus were necessary to invent the process of meiosis that produced gametes
carrying half the number of genes. Sexual reproduction, although complex, time
consuming and costly, was still favoured by animals, so much so that higher
animals have this as the only means of reproduction.
Cost of sexual reproduction
In sexually reproducing populations only 50% individuals produce offspring
while the remaining half just contribute their genes, thus reducing the
reproductive capacity, whereas in asexuals all individuals produce offspring with
no apparent help from others. Sexual reproduction is complex and requires a lot
of energy investment. Individuals must search for a partner and persuade it to
mate, sometimes at great personal risk.
Anisogamy is the most common method of sexual reproduction in which larger
gametes with larger energy investment can be produced in smaller numbers and

smaller gametes that require less energy investment are dispensable and hence
can be produced in large numbers. Larger gametes being a limiting resource set
off fierce competition among the smaller gametes, leading to gametic and sexual
dimorphism and male competition. On the other hand, isogamy which does not
involve such high cost of energy and time, is not so efficient a method and
encounters mechanical and physiological difficulties of cell fusion. Hence,
evolution favoured anisogamy which evolved later.
Basic steps in the origin of sexual reproduction in Protozoa
There was asexual reproduction in the beginning which was fast and simple but
had its disadvantages as it produced clones and could not get rid of harmful
effects of mutation.
 Meiosis evolved as a means of producing haploid individuals and gametes.
 Isogamy produced equal sized gametes as in Elphidium and mycetozoa,
with equal amount of energy investment. But fusion of such gametes was
on the basis of collision of particles and faced physical and physiological
difficulties when they attempted fusion.
 Different mating types originated in ciliates, fungi and algae which
brought about attraction between different mating types and willingness
to exchange genetic material.
 Anisogamy to some extent tried to solve the problem of isogamy by
producing fewer larger gametes with stored energy for the development of
embryo and a large number of smaller and dispensable gametes with high
searching capacity.
 Large number of mating types was reduced to only two types of
individuals, that is, male and female, which was necessary for the
evolution of anisogamy and sexual dimorphism.
 Larger gametes specialized in the storage of nutrients for the embryonic
development, while the smaller gametes specialized in high searching
capacity and fertilization.

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