302 ZOOLOGY FOR MEDICAL STUDENTS chap. 



process, is short and wide and is provided with a remarkable spiral 

 valve, the lining of the canal projecting inwards right to the centre as 

 a broad spiral shelf round which the food is forced to travel as it passes 

 onwards through the intestine. The physiological significance of this 

 organ is obviously to increase the digestive area of intestinal lining with 

 which the food is brought in contact. On the other hand it is also of 

 morphological interest as it appears to be a reminiscence of a time when 

 the primitive vertebrates possessed a ,long intestine coiled into a spiral. 

 The wide part of the intestine containing the spiral valve is succeeded 

 by a short narrow portion — the rectum — and this in turn by the terminal 

 portion or cloaca (cl). 



The primary function of the alimentary canal — -the digestion and 

 assimilation of the food — -necessitates its being provided with glands for 

 the secretion of digestive ferments and other substances concerned in 

 these processes. As in the case of the Earthworm numerous gland-cells 

 are scattered throughout the endodermal lining. Other gland-cells are 

 congregated together in localized outgrowths of the enteric wall, forming 

 bulky glands. Of these the most conspicuous is the liver (Fig. 125, It) 

 which is indeed the most bulky organ in the coelomic cavity. The liver 

 is a compact organ possessing two main lobes, a right and a left, and in 

 the case of Acanthias a small intermediate lobe in addition. It consists 

 of a mass of fine anastomosing tubules and its secretion drains by a 

 duct — the bile-duct — into the intestine close to its front end. Connected 

 with the bile-duct is a reservoir — the gall-bladder — which in Scyllium is 

 embedded in the left lobe of the liver and almost completely hidden 

 from view, and in Acanthias is partially ensheathed in the small inter- 

 mediate lobe. 



From its relation to the alimentary canal there can be little doubt 

 that the vertebrate liver was originally a digestive gland, but although 

 its secretion — the bile — still plays a certain part in digestion, particularly 

 in breaking up fat into finely divided particles, the digestive function 

 has in the modern vertebrate become less conspicuous owing to the rise 

 in importance of other functions. One of these is the excretory function. 

 Various waste materials are extracted from the blood and got rid of in 

 the bile, some of them deeply coloured pigments which give the bile a 

 characteristic yellow or green colour. Again the liver is the chief seat 

 of formation of the important nitrogenous waste substance urea. This 

 is formed in the metabolism of the nitrogenous products of protein 

 digestion brought from the intestine by the blood of the portal vein : 

 it is then passed back into the blood-stream to be carried to the kidneys 

 and there got rid of. Another important function is that of acting as 



