232 AN INTRODUCTION TO ZOOLOGY 



membrane separating the pericardial from the peritoneal cavities. The 

 liver is composed of a large number of ramifying branched tubules, 

 which, as they originate from an outgrowth of the alimentary 

 canal, are of entodermal origin and so constitute a compound 

 tubular gland, although the actual structure of the adult gland is 

 masked by its being tightly bound together by mesodermal connec- 

 tive tissue. Embedded in the front end of the left lobe of the liver 

 is a fairly large thin- walled sac of dark green colour, the gall bladder, 

 in which is stored the bile secreted by the liver. From it comes off 

 a small tube, the cystic duct, which runs between the two main lobes 

 for a short distance, receiving from them several smaller hepatic 

 ducts. After the confluence of these, the main tube, now distin- 

 guished as the common bile duct, runs on to open into the ventral 

 side of the intestine a short distance beyond the pylorus and behind 

 the beginning of the fold of the spiral valve. The liver of Scy Ilium 

 has the same complex functions as it has in Rana. 



The pancreas is a long thin body of a yellowish- white colour and 

 triangular in cross section, lying dorsally to the intestine and pyloric 

 portion of the stomach. At the front end it expands into a small 

 ventral lobe which lies tucked in the bend between the stomach 

 and intestine close to the pylorus. The pancreatic duct runs through 

 the substance of the gland, emerges from the posterior corner of the 

 ventral lobe, enters the intestinal wall below the pylorus, and after 

 running in the wall of the intestine for about half an inch opens into 

 the inside close to the beginning of the spiral valve near the aperture 

 of the bile duct. Like the liver, the pancreas arises as an outgrowth 

 of the alimentary canal, and so is composed of entodermal cells 

 grouped in glandular acini and bound together by mesodermal 

 tissue. Its function is similar to that of the frog. 



Another quite conspicuous very dark glandular-looking 

 mass is attached to the posterior border of the stomach in the form 

 of a U. This is the spleen, and it is not connected functionally 

 with digestion nor developmentally with the alimentary canal, but 

 is mentioned here because it is bound to the stomach by a strong 

 fold of the mesentery. 



From the last part of the oesophagus to the end of the rectum 

 the gut lies more or less freely in the coelom, but is held in position 

 by a reflection of the peritoneum lining the ccelomic cavity. In the 

 embryo this fold is complete from end to end and is known as the 

 dorsal mesentery, since it comes off from the mid-dorsal line of the 

 ccelom. The posterior middle portion of this disappears in the adult, 

 leaving a large anterior part, the mesogaster, supporting the stomach 

 and first half of the intestine and a smaller posterior part, the 

 mesorectum, attached to the rectum and rectal gland. These 



