Protochordata by Tiasha De (University of Calcutta)

It's all about Protochordates. It contains features, classes, orders along with respective examples and diagrams from Young's and Kotpal's book.

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)

2. Hemichordate with STOMOCHORD


Urochordate larva with NOTOCHORD


Cephalochordate with NOTOCHORD

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.

16. General table for Comparison of 3 Protochordata Sub-phyla