120 ZO.OLOGY SECT. 



ventral cartilage (m. v. c.), which may possibly be the median ventral 

 element of the mandibular arch. Lastly, attached to each side of 

 the annular cartilage and passing backwards and downwards, are a 

 pair of tapering, rod-like styliform cartilages (sty, c.). 



The visceral skeleton also differs in a remarkable manner from 

 the ordinary craniate type. It consists of a branchial basket, 

 formed, on each side, of nine irregularly curved vertical bars of 

 cartilage (Fig. 750, br. b. 1 9), the first placed almosTlmme- 

 diately posterior to the styloid cartilage, the second imme- 

 diately in front of the first gill-cleft, the remaining seven just 

 behind the seven gill-clefts. These bars are united together by 

 four longitudinal rods (Ic. 1 4), of which one lies alongside the 

 notochord and is connected in front with the cranium, two others 

 are placed respectively above and below the gill-clefts, while the 

 fourth is situated close to the middle ventral line and is partly 

 fused with its fellow of the opposite side. The posterior vertical 

 bar is connected with a cup-like cartilage (pc. c.), which supports 

 the posterior and lateral walls of the pericardium. The whole 

 branchial basket lies external to the gill -pouches and branchial 

 arteries, not, like typical visceral arches, in the walls of the 

 pharynx. 



The median fins are supported by delicate cartilaginous fin-rays 

 or pterygiophvres, which are more numerous than the myomeres, 

 and lie parallel to one another in the substance of the fin, extending 

 dcfwnwards to the fibrous neural tube. 



\The muscles of the trunk and tail are arranged in myomeres 

 waich take a zigzag course. In the branchial region they are 

 divided into dorsal and ventral bands which pass respectively 

 abbve and below the gill-slits. A great mass of radiating muscle 

 is inserted into the buccal funnel, and the tongue has an ex- 

 tremely complex musculature. 



Digestive Organs. The teeth are laminated horny cones : 

 beneath them lie mesodermal papillae covered with ectoderm 

 which bear a superficial resemblance to the germs of true calcified 

 teeth. The mouth leads into a buccaJ cavity (Fig. 752, m.) formed 

 from the stomodaeum of the embryo, and communicating behind 

 with two tubes placed one above the other : the dorsal of these is 

 the gullet (oes.\ the ventral the respiratory tube (r. t., see below) : 

 guarding the entrance to the latter is a curtain-like fold, the 

 velum (vl.). The gullet bends over the pericardium and enters 

 the intestine (int.) by a valvular aperture. The intestine passes 

 without convolutions to the anus : its anterior end is slightly 

 dilated and is the only representative of a stomach : its posterior 

 end is widened to form the rectum (Fig. 758, r.). The whole 

 of the intestine is formed from the mesenteron of the embryo, 

 and the blastopore becomes the anus, there being no proctodaeum. 

 The lumen of the intestine is semilunar, owing to the presence 



