SCYLLIUM CANICULA 231 



circularly running blood-vessels. The intestine is characterised 

 inside by the development of a very striking structure known as the 

 spiral valve. It is a large fold of the mucosa twisted into a spiral 

 form. Although in lower forms like the lamprey and in the embryo 

 this is only a low fold recalling the typhlosole of the earthworm, 

 save that it is spirally inserted in the gut wall, in the adult Scyllium 

 it is a very wide fold. Its free edges have united in the middle of 

 the intestinal lumen to form a spirally twisted median axis, and so 

 the cavity, instead of being more or less straight, is converted into a 

 fairly narrow spiral of about eight complete turns. If the wall of 

 the intestine be cut, it presents the appearance of a series of imperfect 

 slightly truncated cones with their bases directed backwards and 

 fitting one inside the other. Thus, although the intestine is relatively 

 short, the path actually traversed by the food in passing through it 

 is quite long. Like the typhlosole, this arrangement has two 

 results ; firstly, it retains the food within the gut for a much longer 

 time to enable digestion to be completed, and, secondly, it provides 

 a much larger area over which the food can be absorbed. The same 

 ends are attained in the higher vertebrates by the development of a 

 long, much-coiled intestine. The beginning of the rectum, just 

 behind the level of the bend of the stomach, is marked by the presence 

 of a small reddish club-shaped structure, the rectal gland, attached 

 to its dorsal wall. This body is highly glandular, and has a central 

 duct opening into the beginning of the dorsal wall of the rectum. 

 The function of this rectal gland has not yet been ascertained satis- 

 factorily, but it is probably the homologue of the ccecum of the higher 

 animals, so that the intestine in front of it corresponds to the small 

 intestine of those forms, and ah 1 there is to represent the large 

 intestine is the rectum. In the dogfish the rectum runs straight on 

 into the cloaca, into which also open the excretory and reproductive 

 ducts, as has been pointed out previously. 



The histology of the various parts of the alimentary canal is on 

 the whole similar to that of the frog. In the connective tissue 

 underlying the enteric epithelium are situated a number of small 

 nodules of lymphoid tissue, each enclosed in a fairly definite capsule 

 and known as the lymph follicles. 



In addition to the rectal gland there are connected with the 

 alimentary canal two glands that play important parts in digestion, 

 these are the liver and the pancreas. The liver is a large conspicuous 

 dark brown gland divided up into two main lobes, one on each side 

 of the body, and a much smaller middle lobe lying ventral to the 

 stomach. The three lobes are continuous at the anterior end and 

 bound together and held in position by a strong membrane, the 

 suspensory ligament of the liver, which attaches them firmly to the 



