SHARKS AND SKATES. 



69 



Neither jjremaxillaries nor maxillaries are present, but the jaws are composed of the 

 palato-quadrate and the Meckelian cartilage. Frequently large accessory cartilages 

 (labial cartilages) occur. No membrane bones occur in the composition of the skull. 

 There are no oj)ercular bones, and in all, except the Holocepliali, the gill-clefts are 

 exposed. 



The ])ectoral arch consists of a cartilage on either side, uniting below in the median 

 line, but never becoming connected with the skull. With this pectoral arch the three 

 basal elements of the pectoral fin (protopterygium, mesopterygium, and metaptery- 

 gium) articulate, and these bear the radial cartilages which support the fin rays. The 

 pelvic arch is incomplete, and is represented by a pair of cartilages which sometimes 

 unite in the median line and always occupy an abdominal position. The abdominal 

 fins are the smaller. In the males these fins bear appendages, known as 'claspers,' 

 with a semi-ossified skeleton, and so joined that they are cajiable of motion indepen- 

 dently of the fins. The dispute concerning their function (whether they act as 

 claspers, as intromittent organs, or as perforators) has not been decided. 



In the teeth of the Elasmobranchs considerable differences may be seen. In 

 Chinuera, most rays, and some sharks, they have obtuse crowns, and frequently are so 

 flattened and united that they form a pavement for both jaws, between which crabs 

 and molluscs are readily crushed. In other and the majority of the sharks, the teeth 

 are sharji and conical, and are arranged in rows, the apices pointing backward, so that 

 the prey cannot readily escape when once it passes the opening. These trenchant 

 teeth and their arrangement indicates a rapacious life, and their habits fully accord 

 therewith. 



The oesophagus is wide, .and communicates with a usually large stomach, but this 

 sometimes can hardly be distinguished from the rest of the alimentary tract. The in- 

 testine is short, and just beyond the duodenal portion, which receives the heji.atic and 

 pancreatic ducts, the so-called spiral valve begins. This 

 is a contrivance to increase the surface, and consists of 

 an internal fold, the fixed edge of which in most forms 

 follows the wall of the intestine in a spiral manner, 

 but in Carcharias, Galeocerdo, tSp/wi/na, etc., this edge 

 may be straight, and the free edge in this case is rolled 

 up like a scroll, the axis being parallel to that of the 

 intestine. The rectum is short, and o|iens into a cloaca, 

 which also receives the ducts of the renal and repro- 

 ductive organs. 



The ccelom communicates with the pericardial cav- 

 ity, and, near the vent, with the exterior, by two abdom- 

 inal pores. Liver, pancreas, and sjileen are present. 

 The heart consists of a single auricle, a single ventricle, 

 and a muscular and rythmically contractile conns 

 arteriosus. On the inner walls of the latter are sev- 

 eral rows of semi-lunar valves. In many sharks the 

 first visceral cleft persists through life, and forms a 

 'spiracle,' an opening extending from the upper outside of the head, in front of the 

 suspensorium, to the cavity of the mouth. From this spiraeular cleft, in the early em- 

 bryonic stage, branchial filaments protrude (Fig. 56). 



The brain is much like that of the Teleosts, and is well developed. The optic 



Fig. 56.— Head of embryo dog-fish, the 

 upper portions removed, to show the 

 cranial cartilages ; s, spiraeular cleft; 

 (■/, brnucbial clefts ; b, branchial. 



