XIII 



PHYLUM CHORDATA 



131 



same tissue. Behind the styloid 

 cartilage or hyoid bar (st. p.) 

 is a rod connected below with 

 the subocular arch ; it probably 

 represents the first branchial 

 bar. The tongue is supported 

 by an immense cartilage (m.v.c.), 

 which probably represents the 

 small median ventral cartilage 

 of the Lamprey (Fig. 750, m.v.c.). 

 The branchial basket is quite 

 rudimentary, being represented 

 only by a small irregular car- 

 tilage in the walls of the oaso- 

 phageo-cutaneous duct, and, in 

 Myxine, by a smaller cartilage 

 (Fig. 762, br. b.) on the right 

 side supporting the common 

 external gill-tube. 



The intestine is very wide. 

 The liver consists of two separ- 

 ate portions, the ducts of which 

 unite to form a common bile 

 duct : a gall-bladder is present. 

 The brain differs considerably 

 from that of the Lamprey, espe- 

 cially in the larger hemispheres, 

 absence of lateral ventricles, 

 and smaller mid-brain. The 

 eyes are vestigial and sunk 

 beneath the skin, and the audi- 

 tory organ (Fig. 763) has only a 

 single semicircular canal, which, 

 having an ampulla at each end, 

 probably represents both anterior 

 and posterior canals. 



Bdellostoma has a persistent 

 pronephros in the form of a 

 paired irregularly ovoidal body 

 situated just above the heart : 

 the nephrostomes open into the 

 pericardium. The functional 

 kidney is the mesonephros, and 

 is specially interesting from the 

 fact that it retains in the adult 

 its primitive segmental arrange- 

 ment. The ureter (archi- 



K 2 



