128 ZOOLOGY SECT. 



cartilages or ventral portion of the mandibular arch of other 

 Craniata (see p. 77) ; it is tipped in front by a small median and 

 a pair of still smaller lateral cartilages. Below it is a slender 

 T-shaped median 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. 794, br. b. 1 9), the first placed almost imme- 

 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 the delicate cartilaginous rods 

 already referred to, which are more numerous than the myomeres, 

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

 downwards to the fibrous neural tube. 



The structure of the cartilage is peculiar and varies in different 

 parts ; it has very little matrix. 



The muscles of the trunk and tail are arranged in myomeres 

 which take a zig-zag course. In the branchial region they are 

 divided into dorsal and ventral bands, which pass respectively 

 above and below the gill-slits ; but in the trunk there is no 

 division into dorsal and ventral parts. A great mass of radiating 

 muscle is inserted into the buccal funnel, and the " tongue " has an 

 extremely complex musculature which derives its nerve-supply 

 from the trigeminal. 



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. When worn out they are succeeded by others developed at 

 their bases. The mouth leads into a luccal cavity (Fig. 796, m.) 

 formed from the stomodseum of the embryo, and communicating 

 behind with two tubes placed one above the other : the dorsal of 

 these is the gullet (ces.), the ventral the respiratory tube (r. t., see 



