11 



Class IX.— HEMICHORDATA (Half-chorded)^ 



ChacaotePS. — Worm-like marine forms which breathe like a vertebrate 

 by slits in the neck. A dorsal nerve-cord (like vertebrates), also a ventral 

 one (like invertebrates). A dorsal skeletal rod resembling the notochord 

 of vertebrates (Chordata). e.g. Balanoglossus (previously placed in the 

 class Vermes, group Enteropneusta) ; Cephalodiscus. 



Class X.— VERTEBRATA (Chordata). 



The Vertebrates include the following groups : — Ueochorda, Cephalo- 

 CHOEDA, Cyclostomata, Ichthtopsida (fishes and amphibia), Sadropsida 

 (reptiles and birds), Mammalia,* — all of which agree in the following 

 characters: — (1) A dorsal nerve-cord, usually divisible into brain and spinal 

 cord; (2) An axial skeletal rod — the notochord — which enters into the 

 formation of the backbone of the adult (when the adult possesses one) ; 

 (3) The heart is developed on the ventral blood-vessels, not dorsally as in 

 invertebrates ; (4) there are gill-slits in the neck, which, in the lower verte- 

 brates (i.e. Urochorda, Cephalochorda, Clyostomata, fishes and young amphi- 

 bians), form breathing organs, — but in the higher vertebrates {i.e. reptiles, 

 birds, and mammals) close up in the adult, and are functionless ; (5) the 

 eye (when present) is an outgrowth from the brain (not a mere epidermal 

 structure as in invertebrates) ; (6) bUateral symmetry. 



UEOCHOEDA {Tail-chorded animals; Tunicata; Ascidians; Sea-squirts). 



Characters. — Vertebrates, which in the young stage (larva) resemble, 

 both externally and internally, the frog tadpole — i.e. they have (1) a noto- 

 chord, which, however, is confined to the tail region of the body ; (2) eye; 

 (3) gill-slits in the neck, opening directly to the exterior; (4) well developed 

 tail. The adult (except Appendicnlaria) becomes greatly degraded. It loses 

 its tail, notochord, dorsal nerve-cord, and eye ; becomes fixed (mostly), and 

 develops a mantle around itself somewhat after the manner of the molluscs 

 (hence the name Tunioata for the group), part of which forms an atrial 

 chamber for the reception and escape of the water of respiration. 



* The latter three — lohthyopsida, Sauropsida, and Mammalia — are sometimes called 

 the true Vertebrata (having well developed backbones), or the Craniata (having well 

 developed heads) ; the previous three groups — Urochorda, Cephalochorda, and Cyclos- 

 tomata — being designated Acraniata (head-less). 



