116 THE ANATOMY OF VERTEBRATBD ANIMALS. 



Aetobatis only tlie middle transversely elongated teeth remain. 

 In the Sharks and Rays the teeth are developed from papillfe, 

 or ridges, situated at the bottom of a deep fold within the mu- 

 cous membrane of the jaw. The teeth come to the edge of 

 the jaw, and, as they are torn away or worn down by use, 

 they are replaced by others, developed, in successive rows, 

 from the bottom of the groove. No such successive develop- 

 ment takes place in the Chimwra. 



As in other fishes, there are no salivary glands. The wide 

 oesophagus leads into a stomach which is usually spacious and 

 sac-like, but sometimes, as in Ohimcera, may be hardly distinct 

 from the rest of the alimentary canal. No diverticulum filled 

 with air, and constituting a swimming-bladder, as in Ganoid 

 and many Teleostean fishes, is connected with either the oesoph- 

 agus, or the stomach, though a rudiment of this structure has 

 lately been discovered in some Elasmobranchs. 



The intestine is short, and usually commences by a dilata« 

 tion separated from the stomach by a pyloric valve. This 

 duodenal segment of the intestine is usually known as the 

 JBursa Mitiana. It receives the hepatic and pancreatic ducts, 

 and, in the foetus, the vitelline duct. Beyond this part, the 

 absorptive area of the mucous membrane of the small intes- 

 tines is increased by the production of that membrane into a 

 fold, the so-called spiral valve, the fixed edge of which usually 

 runs spirally along the wall of the intestine. In some sharks 

 ( Garcharias, Galeocerdo) the fixed edge of the fold runs 

 straight and parallel with the axis of the intestine, and the 

 fold is rolled up upon itself into a cylindrical spiral. 



The short rectum terminates in the front part of a cloaca, 

 which is common to it and the ducts of the renal and the re- 

 productive organs. The peritoneal cavity communicates with 

 that of the pericardium in front, and, behind, opens externally 

 by two abdominal pores. The heart presents a single auricle, 

 receiving the venous blood of the body from a sinus venosus. 

 There is a single ventricle, and the walls of the aortic bulb con- 

 tain striped muscular fibres, and are rhythmically contractile, 

 pulsating as regularly as those of the auricle and ventricle. 



The interior of the bulb exhibits not merely a single row 

 of valves at the ventriculo-bulbous aperture, but several other 

 transverse rows of semilunar valves, which are attached to the 

 walls of the bulb itself, and at its junction with the aorta. 

 These valves must be of great importance in giving full effect 

 to the propulsive force exerted by the muscular wall of the 

 bulb. 



