(i) Body divided into three regions-proboscis, collar and trunk. (ii) Notochord doubtful, short, confined to proboscis and non-homologous with that of chordates. Class 1: Enteropneusta (Gr; enteron.gut + pneusta, breathed) (i) Body large, and worm like. (ii) Gill slits are numerous. (iii) Intestine straight.
(iv) Acorn or tongue worms. 70 species Examples: Balanoglosus, Saccoglossus. Class 2: Pterobranchia (Gr; Peteron, feather + brancheon, gill) (i) Body small and compact. (ii) Gill slits one pair or none. (iii) Intestine U shaped. (iv) Pterobranchs, 20 species. Examples: Cephalodiscus, Rhabdopleura etc.
2. Subphylum 2- Urochordata (Gr; Oura, tail + L; chorda, chord) (i) Notochord and nerve cord only in tadpole like larva. (ii) Adult sac like, often sessile and encased in protective tunic. (iii) Commonly called as Tunicates.
Class 1: Ascidiacea: (i) Sessile tunicates with scattered muscles in tunic. (ii) Gill clefts numerous (iii) Ascidians or sea squirts. 12,00 species. Examples: Herdmania, Clona, Molgula. etc.
Class 2: Thaliaceae (i) Free-swimming or pelagic tunicates with circular muscles in tunic. (ii) Salps or chain tunicates. 30 species. Examples: Doliolum, Pyrosoma. etc.
Class 3: Larvaceae (i) Tiny, transparent, free-floating. (ii) Adults retain many larval features including tail. (iii) Only two gill-slits. 30 species. Example: Oikopleura.
3. Subphylum 3- Cephalochordata (Gr; Kephale, head + L; chorda; Chord), (i) Notochord and nerve cord present throughout life along entire length of body. Class 1: Leptocardii (i) Body fish like, segmented with distinct myotomes. (ii) Gill slits are numerous. (iii) Free swimming and burrowing. 30 species.
Example : Branchiostoma (Amphioxus)