The document discusses the phylum Arthropoda, which includes insects, arachnids, crustaceans, and others. It describes their key characteristics such as a jointed exoskeleton made of chitin, jointed limbs, and growth through molting. Arthropods are the largest and most diverse phylum, making up over 75% of animal species, mostly insects. Their success is attributed to traits like metamerism, tagmatization, their exoskeleton, hemocoel circulatory system, and ability to undergo metamorphosis. The phylum contains five subphyla characterized by their body plans, appendages, and habitats.

1. Instructor: Dr Alex Shayo
March 2016

2.  Arthropods are animals
belonging to the Phylum
Arthropoda (from Greek arthron,
"joint", and podos "foot", which
together mean "jointed feet"),
and include the insects,
arachnids, crustaceans, and
others.
 Arthropods are characterized by
their jointed limbs and cuticles,
which are mainly made of chitin.
 The rigid cuticle inhibits growth,
so arthropods replace it
periodically by moulting.
 The Phylum Arthropoda is the
largest and most diverse of all
animal phyla.
 More than three quarters of the
animals on earth are arthropods,
and most of these are insects. 2

3.  They range in size from microscopic plankton up to forms
a few meters long. They have exploited every known
environment
 Despite their great diversity, the basic body plan of
arthropods is fairly constant. In summary, the following
are the characteristics of the phylum Arthropoda:
1. Metamerism modified by specialization of body regions
for specific functions (tagmatization)
2. Chitinous exoskeleton that provides support and
protection and is modified to form sensory structures
3. Paired, jointed appendages
4. Growth accompanied by ecdysis or moulting
5. Coelom reduced to cavities surrounding gonads and
sometimes excretory organs
6. Open circulatory system in which blood is released into
tissue spaces (hemocoel) derived from blastocoel
7. Complete digestive tract
8. Metamorphosis is often present. This reduces competition
between immature and adult stages
3

4.  Four aspects of arthropod biology have contributed
to their success:
1. Metamerism and Tagmatization
2. The Exoskeleton
3. The Hemocoel
4. Metamorphosis
4

5.  Arthropod body is often composed of a series of similar segments, each
bearing a pair of appendages
 Metamerism of arthropods is most evident externally. Internally,
however, septa do not divide the body cavity of an arthropod, and most
organ systems are not metamerically arranged
 Metamerism permits the specialization of regions of the body for
specific functions.
 This regional specialization is called tagmatization. These body regions,
called tagmata (sing. tagma), are specialized for feeding and sensory
perception, locomotion, and visceral functions
 Each tagma of an arthropod, including those fused together, typically
bears a pair of jointed appendages. In the head region, one or two pairs
are modified into long sensory structures called antennae or antennules.
Tooth-like jaw appendages termed mandibles are also usually present.
Maxillae are limbs in the head region that are modified to pass food to
the mouth. Most of the other appendages function as walking or
swimming limbs; in aquatic forms they may have attached gills 5

6.  Enclosing arthropod body is an external, jointed skeleton
called an exoskeleton or cuticle
 The Exoskeleton is often considered the major reason for
arthropod success. It provides structural support,
protection, impermeable surfaces for the prevention of
water loss, and a system of levers for muscle attachment and
movement
 The exoskeleton covers all body surfaces and invaginations
of the body wall, such as the anterior and posterior portions
of the gut tract. It is nonliving and is secreted by a single
layer of epidermal cells
 The epidermal layer is sometimes called the hypodermis
because, unlike other epidermal tissues, it is covered on the
outside by exoskeleton, rather than being directly exposed
to air or water
 The exoskeleton is periodically shed in moulting process
called ecdysis. Ecdysis is controlled by nervous and
endocrine systems
6

7.  The hemocoel is derived from an embryonic cavity called
blastocoel that forms in the blastula. Hemocoel provides an
internal cavity for the open circulatory system of arthropods.
Internal organs are bathed by body fluids in the hemocoel to
provide for the exchange of nutrients, wastes, and
sometimes gases
 The coelom was reduced in ancestral arthropods. The
presence of the rigid exoskeleton and body wall means that
the coelom is no longer used as a hydrostatic compartment.
In modern arthropods, the coelom forms small cavities
around the gonads and sometimes the excretory structures
 In arthropods, "blood" is confined to vessels during only a
portion of its circuit through the body. The remainder of its
journey takes place within the body cavity (called the
hemocoel). For this reason, arthropod blood is called
hemolymph
 The circulation system is composed of tubular hearts and
aorta running along the dorsal side of the arthropod. The
hearts pump hemolymph into the sinuses of the hemocoel
where exchange of materials take place 7

8.  Metamorphosis is a radical change
in body form and physiology as an
immature stage becomes an adult
 The evolution of arthropods has
resulted in an increased divergence
of body forms, behaviours, and
habitats between immature and
adult stages
 Having different adult and
immature stages means that the
stages do not compete with each
other for food or living space
 In some arthropod, larvae also serve
as the dispersal stage
 Metamorphosis can be complete or
incomplete
8

9.  Arthropods are typically
classified into five subphyla:
1. Trilobitomorpha
2. Chelicerata
3. Myriapoda
4. Crustacea (Biramia)
5. Hexapoda (Uniramia)
9

10.  Members of this subphylum were a
dominant form of life in the oceans
from the Cambrian period (600 mya)
to the Carbonferous period (345 mya)
 They crawled along the substrate
feeding on annelids, mollusks, and
decaying organic matter. The
trilobite body was oval, flattened. It
had three longitudinal sections and
thus the subphylum name
 All body segments articulated so that
the trilobite could roll into a ball to
protect its soft ventral surface. The
trilobite appendages consisted of two
lobes or rami called biramous
appendages
 The inner lobe was a walking leg, and
the outer lobe bore spikes or teeth
that may have been used in digging
or swimming or as gills in gas
exchange
 Members are all extinct
10

11.  This subphylum includes familiar animals, such as spiders,
mites and ticks and less familiar animals, such as horseshoe
crabs and sea spiders
 These animals have two tagmata. The prosoma or
cephalothorax is a sensory, feeding, and locomotor tagma. It
usually bears eyes, but unlike other arthropods, never has
antennae. Paired appendages attach to the prosoma. The
first pair, called chelicerae, are often pincerlike or chelate,
and are most often used in feeding. They may also be
specialized as hollow fangs or for a variety of other
functions.
 Posterior to the prosoma is the opisthosoma, which contains
digestive, reproductive, excretory, and respiratory organs
 Subphylum chelicerata, has three classes: class
Merostomata, class Arachnida and class Pycnogonida
11

12. 12

13.  This class includes the extinct giant water
scorpions and only four species of living
horseshoe crabs
 The prosoma is covered by a hard,
horseshoe-shaped carapace.
 Opisthosoma includes a long,
unsegmented telson. The first pair of the
opisthosomal appendages covers genital
pores and is called genital opercula. The
remaining five pairs of appendages are
book gills (The name is derived from the
resemblance of these plate like gills to the
pages of a closed book). Gases are
exchanged between the blood and water
as blood circulates through the book gills
 Horseshoe crabs are dioecious. The male
mounts the female and grasps her with
his pedipalps. The female excavates
shallow depressions in the sand, and as
she sheds eggs into the depressions, the
male fertilizes them. Fertilized eggs are
covered with sand and develop
unattended 13

14.  This class includes spiders, mites, ticks and scorpions. Most of these
archnids are carnivores. They hold small arthropods with their
chelicerae while enzymes from the gut tract pour over the prey. Partially
digested food is then taken into the mouth. Others inject enzymes into
prey through hollow chelicerae (as in spiders) and suck partially
digested animal tissue
 Arachnids use coxal glands and/or Malpighian tubules for excreting
nitrogenous wastes
 The body of an arachnid has a variety of sensory structures. Most
mechanoreceptors and chemoreceptors are modifications of the
exoskeleton, such as projections, pores, and slits, together with sensory
cells
 Arachnids are dioecious. Paired genital openings are on the ventral side
of the second abdominal segment. Sperm transfer is usually indirect.
The male often packages sperm in a spermatophore, which is then
transferred to the female. Courtship rituals confirm that individuals are
of the same species and attract a female to the spermatophore. In some
arachnids (e.g. spiders), copulation occurs, development is direct and
the young hatch from eggs as miniature adults. Many arachnids tend
their developing eggs and young during development 14

15. Many scorpions are ovoviparous; in which development
is internal, although large, yolky eggs provide all the
nourishment for development. Some scorpions,
however, are viviparous, meaning that the mother
provides nutrients to nourish the embryos. After birth,
the young crawl into the mother’s back, where they
remain for up to a month.
Mites and ticks (acarines) are mostly ectoparasites on
humans and domestic animals. Others are free-living in
both terrestrial and aquatic habitats
15

16.  Members of the class
Pycnognida are the sea spiders
 All are marine and worldwide,
but are most common in cold
waters. They live on the ocean
floor and frequently feed on
cnidarian polyps and ectoprocts
 Some sea spiders feed by
sucking prey tissues through a
proboscis. Others tear at prey
with their first pair of
appendages
16

17.  Crustaceans are a very large group of
arthropods. They include various
familiar animals, such as crabs, lobsters,
crayfish, shrimp, krill and barnacles
 The majority of them are aquatic, living
in either marine or fresh water
environments, but a few groups have
adapted to life on land. Crustaceans are
among the most successful animals, and
are as abundant in the oceans as insects
are on land
 Crustaceans differ from other living
arthropods in that; they have two pairs
of antennae, whereas all other
arthropods have one pair or none. In
addition crustacean appendages are
typically biramous, meaning they are
divided into two parts; this includes the
second pair of antennae, but not the
first, which is uniramous. Despite their
diversity of form, crustaceans are united
by the special larval form known as the
nauplius
Uniramous appendage Biramous appendage
17

18. 18
 Mandibles (biting jaws) likely evolved from a pair of
limbs that took on a chewing function
18
• Most crustaceans
have separate sexes
• Majority develop
through a nauplius
stage

19. 19
 Decapod crustaceans include shrimps, lobsters, crabs
and crayfish
 Have ten feet
 Exoskeleton usually enforced with CaCO3
 Most body segments are fused into a cephalothorax
 Lobsters and crayfish have appendages that aid in
swimming
 Swimmerets and uropods
19

20. 20
20

21. 21
 Order Cirripedia
-Barnacles are crustaceans that are sessile as adults
21
-Free-swimming
larvae
-Are hermaphroditic
-Some have stalks

22.  This subphylum is subdivided into four classes:
1. Diplopoda (millipedes)
2. Chilopoda (centipedes)
3. Symphyla (symphylans)
4. Pauropoda (pauropodans)
 They are all characterized by a body consisting of two
tagmata (head and trunk) and uniramous
appendages. All modern myriapods are terrestrial.
22

23.  The class diplopoda (Gr. diploos, twofold +
podus, foot) contains the millipedes.
Millipedes have 11 to 100 trunk segments
derived from an embryological and
evolutionary fusion of primitive metameres.
An obvious result of this fusion is two pairs
the occurrence of of appendages on each
apparent trunk segment. Each segment is
actually the fusion of two segments. Most
millipedes are round in cross section,
although some are more flattened
 Millipedes are worldwide in distribution and
area nearly always found in or under leaf
litter, humus or decaying logs to avoid
desiccation since their epicuticle does not
contain much wax
 Male millipedes transfer sperm to female
millipedes with modified trunk appendages,
called gonopods, or in spermatophores. Eggs
are fertilized as they are laid and hatch in
several weeks. Immature acquire more legs
and segments with each molt until they reach
adulthood 23

24. 24

25.  Members of the class Chilopoda (Gr. cheilos, lip + podus,
foot) are the centipedes. Most centipedes are nocturnal
and scurry about the surface of logs, rocks or other forest
floor debris. They lack a waxy epicuticle and therefore
require moist habitats.
 Their bodies are flattened in cross section, and they
have a single pair of long legs on each of their 15 or more
trunk segments. The last pair of legs is usually modified
into long sensory appendages.
 Centipede reproduction may involve courtship displays
in which the male lays down a silk web using glands at
the posterior tip of the body. He places a spermatophore
in the web, which the female picks up and introduces
into her genital opening. Eggs are fertilized as they are
laid. A female may brood and guard eggs by wrapping
her body around the eggs, or they may be deposited in
the soil. Young are similar to adults except that they have
fewer legs and segments. Legs and segments are added
with each moult
25

26. 26

27.  Members of the class Pauropoda (Gr. pauros, small + podus,
foot) are soft bodied animals with 11 segments. These
animals live in forest floor litter, where they feed on fungi,
humus and other decaying organic matter. Their small size
and thin, moist exoskeleton allow gas exchange across the
body surface and diffusion of nutrients and wastes in the
body cavity
 Members of the class Symphyla (Gr. sym, same + phyllos,
leaf) are small arthropods (2 to 10 mm in length) that occupy
soil and leaf mold, superficially resemble centipedes, and
have 12 leg bearing trunk segments. The posterior segment
may have one pair of spinnerets or long, sensory bristles.
Symphylans normally feed on decaying vegetation; however,
some species are pests of vegetables and flowers
27

28.  There are a huge amount of body shapes, but
the "basic insect" body plan is a head, thorax,
abdomen, one pair of antennae, and
mouthparts in three parts (mandibles,
maxillae and a labium)
 The head has the mouthparts and most of the
sensory organs including the eyes and
antennae. The thorax is subdivided into
prothorax, mesothorax and metathorax; each
bears a pair of legs - never more than 3 pairs
in adult insects, and 2, 4 or no wings. The
thorax is simply a box of muscles linked to the
legs and wings. There are eleven or fewer
segments in the abdomen and they contain
the organs for digestion, excretion and
reproduction
 The internal organs are bathed in
haemolymph which transports waste and
nutrients and is moved around the body by a
tube shaped heart and the movement of the
insect. Adult insects have six legs, but larvae,
e.g., caterpillars, maggots may have fewer or
more than six legs (caterpillars have six true
legs and up to ten prolegs). Most have three
simple eyes as well as compound eyes
28

29.  The secret of the success of the insects can be linked to the following
features:
i. Tagmosis allowing specialization in each area
ii. Jointed appendages and striated muscles giving efficient locomotion
iii. Exoskeleton which provides protection and prevents dehydration
while still being flexible and light enough to allow movement. The
hardening of the cuticle is a result of sclerotization, i.e. the bonding
of protein molecules and their cross-links within and between the
lammellae of the procuticle. This forms sclerotin, a resistant and
insoluble protein
iv. Metamorphisim reducing competition between individuals of
different stages in the life cycle, e.g. caterpillar and butterfly occupy
different niches
v. Flight : Insects were the first animals to fly. The evolution of wings
greatly helped in the dispersal and radiation of insects. The wings are
a double cuticular membrane containing veins which provide
structural stability
29

30.  Insects have complex sensory organs of vision, chemoreception, hearing and
touch; all enhancing awareness of surroundings. Insects detect smells
mainly using their antennae. The antennae are also used to touch, taste and
in some cases, to detect sound. Hormones regulate many insect functions,
including ecdysis and metamorphosis. Pheromones are chemicals emitted
by one individual that alter the behaviour of another member of the same
species
 Malpighian tubules attach to mid-gut and serve in elimination of wastes.
Nitrogenous wastes are converted to uric acid to conserve water
 One more reason for insects’ success is their high reproductive potential.
There are modifications for mating and ovipositioning (egg laying). The
"claspers" seen on the final segments of many male insects, e.g. locusts are
used to hold the female while mating. Ovipositors allowing females to lay
eggs in concealed sites and in hosts e.g. parasitic wasps; in social female
bees and wasps this has been adapted to form a sting.
There are also modifications to attract a mate, e.g. stridulatory
mechanisms (sound production by rubbing one part of the body against
another), pheromone release and antennal modifications, and
bioluminescence. All these increase reproductive potential of insects
30

31. There are about 30 described
orders of insects and since
insects are so diverse, more
detailed study is reserved for
a special branch of biology
called Entomology (insect
science).
31

32. 32

33. Phylum Echinodermata is the great group that
contains such familiar marine forms as sea stars,
sea urchins, britlestars, sea cucumbers and sea
lilies.
The name Echinodermata means ‘spiny-skinned’
animals reflecting the projecting spines or
tubercles that give a rough, warty appearance to the
surface of most echinoderms. These spines protect
the echinoderms from their predators. Their
morphologic organization is unique in the animal
kingdom, and their structural peculiarities tend to
separate them from all other phyla.
Echinoderms are exclusively marine. The phylum
contains about 7,000 living species. It is the largest
phylum that has no freshwater or terrestrial
representatives. 33

34.  Of all invertebrate groups, they are placed the closest to the
chordates and represent the most advanced of the
invertebrate phyla within the Deuterostomia, to which the
chordates belong.
 Modern echinoderms have a form of radial symmetry, called
pentamerous radial symmetry, in which body parts are
arranged in fives, or a multiple of five, around an oral-aboral
axis.
 Radial symmetry is adaptive for sedentary or slowly moving
animals because it allows a uniform distribution of sensory,
feeding, and other structures around the animal. Fossil and
larval forms are however bilaterally symmetrical. Some
modern mobile echinoderms have secondarily returned to a
basically bilateral form.
 In some respects their physiologic process are very simple
because they have no head, no excretory system, no methods
of copulation and no internal fertilization. Many of their
body systems, such as the respiratory, reproductive,
circulatory and nervous systems are organized on a simple
plan.
34

35.  The digestive system is relatively short, and digestive glands
are located within the arms of the starfish, or on the outer
body wall of the other members.
 Digestion, however, is a bit different than other animals, in
that, much of it takes place outside the body of the starfish.
The echinoderm will put its stomach outside of its body and
begin to digest its prey before it takes it in to finish the
process.
 Sex organs are also located in the arms of the starfish, and
echinoderms are capable of both sexual and asexual
reproduction.
 The regeneration properties of starfish allow them to regrow
lost limbs as well as the limb regrowing a body. Respiratory
organs, called skin gills, are found throughout the body wall
providing the echinoderm with oxygen. 35

36.  Like other radially symmetrical animals, the echinoderms have a
non centralized nervous system, a feature allowing them to engage
their environments equally from all directions.
 Much of the biology of the echinoderms is associated with their
unique water vascular system.
 The water vascular system is a complex of fluid filled canals and
reservoirs that aid in internal transport and hydraulically operate
fleshy projections called tube feet. The external parts of the tube
feet, or podia, serve a variety of functions including locomotion,
gas exchange, feeding, attachment and sensory perception.
 An organ called an ampulla pumps water into the tubes
increasing the pressure in tube feet causing it to expand and
create suction on a nearby surface. As the pressure on the tube
foot decreases, the tube contracts without decreasing the suction,
and the echinoderm is pulled in that direction.
36

37. 37
Echinoderm Water vascular system

38. Water enters through sieve plate or madreporite on
aboral surface into a short, straight stone canal. Stone
canal connects to a circular canal around the mouth
called the ring canal. Five radial canals extend down
each arm & are connected to the ring canal. Radial
canals carry water to hundreds of paired tube feet.
Bulb-like sacs or ampulla on the upper end of each tube
foot contract and create suction to help move, attach, or
open bivalves. Rows of tube feet on oral surface
(underside) are found in ambulacra grooves under each
arm.
38

39.  Some echinoderms are carnivorous (for example starfish)
others are detritus foragers (for example some sea
cucumbers) or planktonic feeders (for example basket
stars).
 Reproduction is carried out by the release of sperm and eggs
into the water. Most species produce pelagic (= free floating)
planktonic larvae (called bipinnaria or dipleurula larva)
which feed on plankton. These larvae are bilaterally
symmetrical, unlike their parents. When they settle to the
bottom they change to the typical echinoderm features.
 Echinoderms have evolved an internal skeleton
(endoskeleton) made of numerous calcium plates. In fact,
echinoderms were the first animals to have endoskeletons.
The endoskeleton lies just below the outer epidermis and
functions to protect the internal organs.
39

40.  Living echinoderms are classified into five classes:
1. Class Crinoidea (sea lilies/ feather stars)
2. Class Asteroidea (starfish),
3. Class Ophiuroidea (brittle stars or snake stars)
4. Class Echinoidea (sea urchins and sand dollars)
5. Class Holothuroidea (sea cucumbers)
40

41.  These are elegant echinoderms with small, soft
bodies that are surrounded by 10 or more
elongate and upwardly raised arms, that each
comprises of a central axis and then numerous
side branches (pinnules)
 Food particles are captured by the arms and are
passed along ciliated grooves back to the mouth
(situated on the upper surface). Most of the
body cavity is filled by a plain U-shaped gut that
ends in a cone in close vicinity to the mouth.
The gonads are situated in the pinnules closest
to the body and they swell greatly in size during
the breeding season, and ultimately tearing and
releasing the sperm or eggs into the water.
 The Crinoids are essentially sedentary but they
have the ability to crawl or even swim with the
use of their arms. Attached stalked crinoids (sea
lilies) are abundant in the fossil record but very
few species survive today. 41

42.  The starfish are probably the most familiar
echinoderms. They are identified by their flattened
bodies which merge into five (sometimes more)
thick and fleshy, tapering arms.
 If one of these arms were to break off, the animal
would quite simply grow a new one and the lost arm
will simply grow itself a new animal. The mouth is
located on the central part of the under surface of
the body and the anus in turn is located on top
 Each one of the arms contains their own respiratory,
digestive and reproductive organs. A groove that
runs from the mouth along the underside of each
arm protects rows of tiny hydraulically-operated
tubefeet with suckered tips. These tubefeet are
responsible for the creeping movements of the
starfish.
 Some species of Astroidea feed on detritus or
microalgae, but many are predators. Starfish can
evert (turns inside out) its stomach through its
mouth and inserts it into prey. Stomach secretes
enzymes to partially digest the prey then the
stomach is withdrawn and digestion completed
inside starfish.
42

43.  Brittle stars consist of a flat, circular body (disc)
with five or more long thin arms that perform
snakelike movements. These arms break of
extremely easily, hence the name "Brittlestars".
 The sides of the arms are often spiny. They move
by the snake-like undulation of their legs. The
mouth is situated at the lower surface and is
surrounded by five toothed jaws. The texture of
the disc allows division of the brittlestars into 3
groups:
1. Those with granules
2. Those with short spines
3. Those with a leathery or scaley texture.
 Most of the brittle stars have minute planktonic
larvae, but a few brood their young in their bodies
and give birth to extremely small copies of
themselves. Most of the species are detritus
feeders.
43

44.  Most have a globular body that is encased in a hard
calcium carbonate shell (the test). The test is formed by
the fusion of the spines in the skin. The mouth occurs
centrally on the underside, with the anus usually on the
upper side.
 Through tiny pores in the test, tube feet protrude (five
double rows of tube feet run from the apex down the sides
of the test) and serve for movement for the sea urchin.
Long protective spines project from the test.
 In most of the cases the spines are harmless to humans,
but if stung by one it can be irritating. A few species have
long lance-like spines with backwards pointing
serrations. They can penetrate very deeply and then break
of. Some of these longspined urchins discharge poisons
through the tips of their broken arrows.
 Most Urchins are grazers (the flattened sand-dwelling
species feed on detritus), and the more mobile species
control the growth of the seaweed population. In the
topics where there are more predatory fish, most of the
urchins shelter in crevices and feast upon drift-weed.
Some are even able to burrow into the rock, creating holes
in which they shelter 44

45.  The cucumbers have traded in their star shaped, radial
symmetry and skeletal structure, common to other
echinoderms, for an elongate sausage-shaped body with
a leathery skin that lies on its side.
 Up to five rows of tube feet that run along the side is the
only reminder of their ancestral radial symmetry. They
have also become a lot more flexible with their spines
being reduced to microscopic spicules. A mouth that is
surrounded by 10-20 retractable feeding tentacles is
situated on one end, with the anus on the other. Their
sticky tentacles are used to gather detritus or to catch
plankton that floats overhead.
 When some species are disturbed they employ special
tactics in order for them to survive. Some species eject
long sticky threads from the anus, others disgorge part or
the entire digestive canal, which they subsequently
regenerate while the intruder is left with the feast.
45

46. 46