Outline of classification of protochordates

Protochordate is an informal category of animals. This group is composed of the Phylum Hemichordata and the Subphyla Urochordata and Cephalochordata.

1. Protochordata : General characters and classification up to orders By, Dr. Manoj K. Bangadkar

2. Protochordata The organisms belonging to the Protochordata are generally known as the lower chordates. They don’t form a “proper” taxonomic group and are only classified as such for convenience purposes. However, they do form a major division of Chordata . They are also known as Acraniata because they lack a true skull. On the basis of the notochord present, they are divided into three sub-phyla- Hemichordata , Urochordata and Cephalochordata .

3. Characteristics of Protochordata They are generally found in marine water. Their body is bilaterally symmetrical, triploblastic , and coelomated . At a certain stage of their lives, their body develops a long, rod-like structure for support called notochord. They exhibit organ system level of organization. Eg ., Herdmania , Amphioxus.

4. Classification of Protochordata

5. HEMICHORDATA Marine, Solitarily Body is divided into proboscis, collar, and trunk. Bilaterally symmetrical Primitive notochord restricted to proboscis only, thus called stomochord . Triploblastic True coelom present Straight or U shaped gut with anus Nervous system normally diffuse and variable Open circulatory system Blood has no colour and corpuscles. Excretory organ- Glomerulus Filter feeding Four classes- Enteropneusta , Pterobranchia , Planctosphaeroidea , Graptolita Eg ., Cephalodiscus , Rhabdopeura

6. Classification of Hemichordata

7. Class I – Enteropneusta (Acorn worms) General Characters More than 70 species 2-2.5 m long Marine, shallow water, solitary Body divided into Proboscis, Collar and Trunk Well developed gill slits and stomochord Dorsal strand of nerve cells- precursor of dorsal hollow nerve cord Reproduction- sexual Live in burrows Filter feeding

8. Class II – Pterobranchia General Characters Tiny, deep sea, colonial No trace of dorsal nerve cord or notochord Proboscis is modified into shield Collar is modified into tentacles Trunk is short and sac like Asexual reproduction by budding is common One or no pharyngeal slits Body covered with protein collagen

9. Class III – Planctosphaeroidea General Characters Small, rounded, transparent and pelagic larvae Larval body covered with branched ciliary bands Elimentary canal ‘L’ shaped

10. Class IV – Graptolita General Characters Exctinct class Fossil graptolites (e.g. Dendrograptus ) were abundent in Ordovician and Silurian period. Tubular chitinous skeleton Colonial habits

11. UROCHORDATA (TUNICATA) Tail chordate Most primitive of the chordates They are found in the marine environment. They are sessile and filter-feeders. They are also known as tunicates because their body is surrounded by a leathery sheath composed of tunicin (cellulose). Retrogressive metamorphosis : The notochord appears in the larval stage in the tail of the larva and disappears in the adult. Triploblastic U shaped gut, Coelom absent Hermophroditic Respiration occurs through gills. They have an Incomplete closed circulatory system. Haemocyanin as blood pigment The excretory organs are absent. They reproduce asexually by budding. Eg ., Herdmania , Selpa

12. Clasification of Urochordata

13. Class I - Ascidiacea Sea squirts Notochord and post anal tail found in the larval stage only Marine, solitary or colonial, Sessile adults Tunic composed of an acellular matrix of tunicin Two opening- an inhelant and exhalent siphon Hermaphroditic Filter feeders Two orders- I) Enterogona and II) Pleurogona

14. Order I- Enterogona : Body divided into thorax and abdomen, Neural gland ventral to ganglion, Single gonad lying behind intestinal loop, larva with two sense organ ( Ocelli and Otolith ), Ex. Ascidia Order II- Pleurogona : Body compact (Undivided), Neural gland dorsal or lateral to ganglion, Gonad two or more embedded in mantle wall, larva with otolith , ex. Herdmania

15. Class II - Thaliacea Small barrel shaped animals Filter feeders Inhalent and exhalent siphon at opposite ends of body Two generations of life cycle- one solitary and other form chain like colonies Three orders- I) Pyrosomida , II) Doliolida and III) Salpida

16. Order I – Pyrosomida :- No free swimming larval stage, ex. Pyrosoma Order II – Doliolida :- Larva with notochord present, ex. Doliolum Order III- Salpida :- Tailed larva absent, ex. Salpa

17. Class III – Larvacea ( Apendicularis ) Larvaceans or Apendicularians Planktonic Oval trunk and long thin tail Tail contain notochord which retain all through the life Body covered with gelatimous layer but not tail. Two orders- I) Endostylophora and II) Polystylophora Order I - Endostylophora : House bilaterally symmetrical with two apertures, pharynx with endostyle , ex. Appendicularia Order II – Polystylophora : House bilaterally symmetrical, single aperture, pharynx without endostyle , ex. Kawalevskia

18. CEPHALOCHORDATA Most advanced protochordates Notocord : W ell developed, persist throughout the life and runs from tail to head (Head cord) Gill slits: Numerous ( Upto 100) Dorsal nerve cord Post anal tail Marine Fish like appearance External fertilization Metamerism found Closed blood circulatory system Haemocyanin pigment Single class- Leptocardii / Leptocardia

19. Class I – Leptocardii / Leptocardia (Small heart) No true heart- A single pulsating vesel Colorless blood No brain, organ of excretion and limbs Backbone is notochord Ex. Amphioxus Figure: Amphioxus