124 ZOOLOGY SECT. 



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, and is only doubtfully related to it. 

 It consists of a branchial basket, formed, on each side, of nine 

 irregularly curved vertical bars of cartilage (Fig. 804, br. b. 1 9), 

 the first placed almost immediately posterior to the styloid cartilage, 

 the second immediately 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 along- 

 side 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 buccal cavity (Fig. 806, 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 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. 812, r.). The whole of the 

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



