12 



SXJB-DIVISIONS. 



1. Pbrennichordata. — Urochords which remain at the larval stage 



throughout life — i.e. they retain the dorsal nerve-cord and notochord 

 of the larva, e.g. Appendicularia. 



2. Caducichordata. — Urochords which become thoroughly degraded, as 



described above. Some remain solitary throughout life. e.g. Asoidia 

 (common sea-squirt) ; Phallusia. Some become fused into a colony 

 or a chain of colonies. 



CEPHALOCHOEDA (Head-chordad). 



Characters. — Vertebrates in which the notochord remains in the adult 

 and extends into the so-called head region. The dorsal nerve-cord is slightly 

 swollen at anterior end to form a rudimentary sort of brain. Eespiration 

 by numerous {i.e. not four or five as in fishes, but about a hundred on each 

 side) slits in the walls of the oesophagus, which open not directly to the 

 exterior, but into a chamber (atrial chamber) which communicates with the 

 exterior by a single aperture. There are no limbs, and practically no head. 

 e.g. Amphioxus (Lancelet) the only genus. 



CYCLOSTOMATA (Round-mouthed). 



Characters. — Eish-like vertebrates, without jaws. Notochord persists, 

 and forms the axial skeleton of the adult, as in Amphioxus. Eespiration 

 by branchial pouches. Kidneys worm-like and primitive. Limbs absent. 

 e.g. Hagfish (Myxine) ; breathes by six pouches on each side of the oesophagus. 

 Each pouch opens directly into the oesophagus at one end, and at the other 

 the pouch is continued into a duct, which runs backwards and then opens to 

 the exterior. All the ducts of the six pouches of each side have a common 

 opening to the exterior. The single nostril opens into the oesophagus, and 

 supplies respiratory water to the breathing pouches. Hagfish is parasitic 

 on fishes, being the only true vertebrate parasite. Lamprey (Petromyzon) 

 breathes by seven branchial pouches, which open directly to the exterior, 

 and internally open into a tube lying under the oesophagus, which com- 

 municates directly with the mouth. 



ICHTHYOPSIDA. 



Characters. — Eespiration by giUs during part or whole of life. Heart 

 never more than three -chambered, and always two aortic arches at least 

 given off from it. The lower jaw is mostly compound, and does not 

 articulate directly with the base of the skull, but is suspended by the 

 quadrate or hyomandibular, or both. Cold-blooded. Sub-divided into — 



