THE ELASMOBRANCH FISHES 219 



in the tough fibrous dermis, and are not fused to or implanted 

 in the jaws. The mouth leads into a wide buccal cavity, 

 of which the flat roof is formed by tr^e floor of the cranium, 

 and the floor is supported by the basihyal cartilage. The 

 buccal cavity passes without any diminution of its size into 

 the pharynx, a region defined by the presence of the internal 

 openings of the spiracles and the five pairs of gill-slits. Passing 

 these by for the moment, we find the pharynx contracting 

 to form an oesophagus, which shortly widens out into a 

 sac-like stomach. The further part of the stomach is bent 

 forwards, and lies alongside of the nearer half, so that the 

 whole organ is U" sna P e ^- The distal end of the stomach 

 is separated by a slight thickening from the intestine, which 

 runs nearly straight backwards in the abdominal cavity, is 

 dilated in its middle portion, and narrows posteriorly to 

 form the rectum, which, after a short course, opens into 

 the cloaca (see fig. 51). The mucous membrane lining the 

 stomach is thrown into longitudinal folds, and the internal 

 surface of the intestine is increased by the spiral valve, a 

 fold of the mucous membrane, one edge of which is attached 

 to the inner wall of the intestine, and runs round it in a 

 spiral of some seven or eight turns. The line of attachment 

 of the spiral valve is marked externally by blood-vessels. 

 Shortly behind the pylorus, the intestine receives the ducts 

 of the liver and pancreas. The liver is very large, consisting 

 of two elongate lobes lying right and left of the stomach, 

 and a much smaller median lobe lying ventral to the stomach. 

 The three lobes are united anteriorly, and suspended from 

 the anterior wall of the abdominal cavity by a median suspen- 

 sory ligament. The gall bladder is a conspicuous oval sac 

 imbedded in the anterior end of the left lobe of the liver. 

 The bile duct passes back from it, receives other ducts from 

 the lobes of the liver, and runs in the membrane connecting 

 the liver with the stomach to the anterior end of the intestine. 

 It runs for a short distance in the wall of the intestine, and 

 opens into it a little in front of the commencement of the 

 spiral valve. 



The main part of the pancreas lies between the two limbs 

 of the stomach. It is a yellowish-white organ, compressed 

 from side to side, and produced at its anterior end into a 

 ventral lobe which lies in the angle between the stomach and 



