Protochordata by Tiasha De (University of Calcutta)
1. PROTOCHORDATA (ACRANIA)
The lower primitive chordates are collectively known as the
'protochordates' or Protochordata (Gr., protos, first; chorde, cord).
They comprise of three major subdivisions ranked as subphyla:
✓ Hemichordata
✓ Urochordata
✓ Cephalochordata
This division is chiefly based on the notochord found in the three
subphyla.
▪ In Hemichordata, the notochord is of doubtful existence. It is
represented by a stomodaeal diverticulum or stomochord to
the proboscis. For this reason, the Hemichordata is now-a-days
treated as an independent invertebrate subphylum.
▪ The Urochordates have notochord only in the tail region of the
tadpole larva.
▪ Only Cephalochordata have a persistent notochord extending
along the whole length of their body throughout life.
Nerve Cord in Hemichordates Notochord in Urochordates
(Diagram from Young’s Book)
4. Sub Phylum 1:
HEMICHORDATA
General Characteristics:
❖ HABITAT: Exclusively marine, solitary or colonial, mostly tubicolous.
❖ STRUCTURE: Body soft, fragile, vermiform, unsegmented,
bilaterally symmetrical and triploblastic.
❖ DIVISION: Body typically divided into 3 distinct regions: proboscis,
collar and trunk.
❖ Body wall of a single layered, epidermis with mucous glands. No
dermis.
❖ Coelom enterocoelous, usually divided into protocoel, mesocoel
and metacoel, corresponding to 3 body regions.
❖ Digestive tube complete, straight or U-shaped.
❖ Foregut gives out a hollow buccal diverticulum into proboscis,
earlier considered as 'notochord'.
❖ Dorso-lateral pharyngeal gill slits, when present, one to several pairs.
❖ Pharynx with endostyle. They are ciliary filter feeders.
❖ Circulatory system simple and open, including a dorsal heart and
two longitudinal vessels, one dorsal and one ventral.
❖ Excretion by a single proboscis gland or glomerulus connected to
blood vessels.
❖ Nervous system primitive consisting mainly of a sub-epidermal
nerve plexus. Dorsal collar nerve cord hollow.
❖ Reproduction is mainly sexual. Sexes are usually separate. Gonads
one to several pairs.
❖ Fertilization external, in sea water. Development direct or indirect
with a free-swimming tornaria larva.
EXAMPLES: Balanoglossus sp. (Acorn Worm), Cephalodiscus sp., etc.
Balanoglossus sp. (From Young’s Book)
5. Balanoglossus sp. (Another diagram)
CLASSes :
According to J.Z. Young (1981), this phylum is divided into 2 classes:
I. Enteropneusta
Balanoglossus sp., Glossobalanus sp., Ptychodera sp.,
Saccoglossus sp.
II. Pterobranchia
Cephalodiscus sp., Rhabdopleura sp.
CLASS 1- ENTEROPNEUSTA
(Gr. enteron, gut + pneustos, breathed)
General Characteristics:
❖ Solitary, free-swimming or burrowing animals, commonly called
the 'acorn' or 'tongue worms'.
❖ Body elongated, vermiform, with no stalk.
❖ Proboscis cylindrical and tapering.
❖ Collar without ciliated arms (lophophore).
❖ Alimentary canal straight. Mouth and anus at opposite ends. Filter
feeding.
6. ❖ Several pairs of U-shaped gill-slits.
❖ Sexes separate. Gonads numerous, sac like.
❖ Development includes tornaria larva in some. Asexual
reproduction lacking.
EXAMPLES: Balanoglossus sp., Saccoglossus sp. (= Dolichoglossus),
Protoglossus sp., Ptychodera sp., Spengelia sp.
7. CLASS 2 – PTEROBRANCHIA
(Gr. pteron, feather + branchion, gill)
General Characteristics:
❖ Solitary or colonial, sessile and tubicolous animals living inside
secreted chitinous tubes.
❖ Body short, compact, with stalk for attachment.
❖ Proboscis shield like.
❖ Collar bearing ciliated arms (lophophore).
❖ Alimentary canal U-shaped. Anus dorsal lying near mouth. Ciliary
feeding.
❖ Gill slits one pair or absent, never U-shaped.
❖ Sexes separate or united. Gonads one or 1 pair.
❖ Development direct or with a larval stage. Asexual reproduction
by budding in some.
EXAMPLES: Rhabdopleura sp., Cephalodiscus sp., Atubaria sp.
8. ❖ The class Pterobranchia has 2 orders:
✓ Rhabdopleurida
✓ Cephalodiscida
(Orders are not in syllabus)
Sub Phylum 2:
uroCHORDATA
General Characteristics:
❖ Exclusively marine and cosmopolitan, found in all seas and
at all depths.
❖ Mostly sedentary (fixed), some pelagic or free-swimming.
❖ Simple (solitary), aggregated in groups or composite
(colonial).
❖ Size (0.25 to 250 mm), shape and colour variable.
❖ Adult body degenerate, sac-like, unsegmented, without
paired appendages and usually without tail.
❖ Body covered by a protective tunic or test composed largely
of tunicine, (C6H10O5)n, similar to cellulose, hence the name
Tunicata.
❖ A terminal branchial aperture and a dorsal atrial aperture
usually present.
❖ Coelom absent. Instead, an ectoderm-lined atrial cavity
present, which opens to outside through atrial aperture.
❖ Notochord present only in larval tail, hence the name
Urochordata.
❖ Alimentary canal complete. Pharynx (branchial sac) large,
with endostyle and two to several pairs of gill-slits. Ciliary
feeders.
❖ Respiration through test and gill-slits.
❖ Blood-vascular system open. Heart simple, tubular and
ventral. Flow of blood periodically reversed. Special
vanadocytes in blood extract vanadium from sea water.
❖ Excretion by neural gland, pyloric gland and nephrocytes.
❖ Dorsal tubular nerve cord only in larval stage, reduced to a
single dorsal nerve ganglion in adult.
❖ Mostly hermaphrodite. Fertilization cross and external.
9. ❖ Development indirect including a free-swimming tailed
larva with basic chordate characters.
❖ Metamorphosis retrogressive.
❖ Asexual reproduction by budding common.
EXAMPLES: Ascidia sp., Ciona sp., Herdmania sp. (Sea Squirt), Botryllus sp.,
Malgula sp., Doliolum sp., Salpa sp., Pyrosoma sp., Oikopleura sp.
CLASSes :
According to J.Z. Young (1981), this phylum is divided into 3 classes:
I. Ascidiacea
Ascidia sp., Ciona sp., Herdmania sp. (Sea Squirt), Botryllus sp.,
Malgula sp.
II. Thaliacea
Doliolum sp., Salpa sp., Pyrosoma sp.
III. Larvacea
Oikopleura sp., Kowaleviskia sp.
CLASS 1- ASCIDIACEA
General Characteristics:
❖ Solitary, colonial or compound. Bottom living.
❖ Body form and size variable.
❖ Test permanent, well developed and thick.
❖ Atrium opens dorsally by atriopore.
❖ Pharynx large with many persistent gill-slits.
❖ Sexes united. Larva free-swimming and highly developed.
❖ Adults usually sessile after retrogressive metamorphosis
when larval notochord, nerve cord and tail are lost and
brain reduced to a solid dorsal ganglion.
❖ Stolon simple or none.
EXAMPLES: Ascidia sp., Ciona sp., Herdmania sp. (Sea Squirt), Botryllus sp.,
Malgula sp., Doliolum sp.
10. Diagram of Herdmania sp. from Young’s book.
Ciona sp. Botryllus sp.
❖ The class Ascidiacea has 2 orders:
✓ Enterogona
✓ Pleurogona
(Orders are not in syllabus)
11. CLASS 2- THALIACEA
General Characteristics:
❖ Adults free living, pelagic, in warm and temperate seas. Solitary or
colonial.
❖ Body shape and size variable.
❖ Tunic permanent, thin and transparent, with
❖ circular muscle bands.
❖ Atriopore located posteriorly.
❖ Pharynx with 2 large or many small gill-slits.
❖ Sexes united. Larva formed or absent.
❖ Adult without notochord, nerve cord and tail.
❖ Asexual budding from a complex stolon.
❖ Life history with an alternation of generations.
EXAMPLES: Doliolum sp., Salpa sp., Pyrosoma sp.
Pyrosoma sp.
12. Salpa sp.
❖ The class Thaliacea has 3 orders:
✓ Pyrosomida
✓ Doliolida
✓ Salpida
(Orders are not in syllabus)
13. CLASS 3- LARVACEA (=Appendicularia)
General Characteristics:
❖ Small (5 mm long), solitary, free-swimming, pelagic,
neotenic, larva-like forms with persistent tail, notochord,
nerve cord and brain.
❖ Test forming a temporary house, renewed periodically.
❖ Atrium and atrial aperture absent.
❖ Gill-slits 2, opening directly to outside.
❖ Sexes united. No metamorphosis.
EXAMPLES: Oikopleura sp., Kowaleviskia sp.
Oikopleura sp.
❖ The class Larvacea has 2 orders:
✓ Endostylophora
✓ Polystylophora
(Orders are not in syllabus)
14. Sub Phylum 3:
CephaloCHORDATA
General Characteristics:
❖ Marine, widely distributed in shallow waters.
❖ Mostly sedentary, and buried with only anterior body end,
projecting above bottom sand.
❖ Body small, 5 to 8 cm long, slender, fish-like, metameric
and transparent.
❖ Lacks head. Body has trunk and tail.
❖ Lacks paired appendages. Median present.
❖ Exoskeleton absent. Epidermis single layered.
❖ Muscles dorso-lateral, segmented into myotomes.
❖ Coelom enterocoelous, reduced in the pharyngeal region.
❖ Notochord rod-like, persistent, extending from rostrum to
tail.
❖ Digestive tract complete.
❖ Pharynx large, perforated by numerous persistent gill-slits.
❖ Filter feeders.
❖ Respiration through general surface.
❖ Circulatory system well-developed, closed and without
heart and respiratory pigment.
❖ Hepatic portal system developed.
❖ Excretion by protonephridia and solenocytes.
❖ Nerve cord dorsal, tubular, without ganglia and brain.
❖ Sexes separate. Gonads numerous and metamerically
repeated.
❖ Sexual reproduction.
❖ Fertilization external, in sea water.
❖ Development indirect, including a free-swimming larva.
EXAMPLES: Branchiostoma sp., Asymmetron sp.
CLASSes :
According to J.Z. Young (1981), this phylum is divided into 1 class:
I. Leptocordii
Branchiostoma sp., Asymmetron sp.