ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



ort and wide and is provided with a remarkable spiral 



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



a Im.ad spiral shelf round which the food is forced to travel as it passes 



ii-ds through the intestine. The physiological significance of this 



m 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 

 tin- 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 (d). 



The primary function of the alimentary canal the digestion and 



nilation 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 



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



g negated 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 Acanihias 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 1 tile-duct is a reservoir the gall-bladder which in Scylliwn is 

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

 1'roni view, and in Acanthias is partially ensheathed id 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 



< ret ion the bile still plays a certain part in digestion,, particularly 



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



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 



harart eristic yellow or green colour. Again the liver is the chief seat 



rmation o! the important nitrogenous waste substance urea. This 



>rmcd in the metabolism of the nitrogenous products of protein 



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



'] back into the blood-stream to be carried to the kidneys 



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



