266 ELEMENTARY ZOOLOGY 



The above operation will have severed the large power- 

 ful muscles forming the body-wall and extending along 

 the sides. Note a membranous sac completely filling a 

 large dorsal cavity. This is the swim- bladder, a float 

 filled with air which tends to give the fish the same weight 

 as the water it displaces. It arises as a diverticulum from 

 the alimentary canal, but soon becomes permanently shut 

 off from it. Beneath the swim-bladder is a large cavity 

 filled with various organs, collectively known as the 

 viscera. In vertebrate animals the cavity which contains 

 the viscera is generally called the peritoneal cavity. It is 

 lined by the peritoneum, a delicate membrane, part of 

 which is deflected as the mesentery over the alimentary 

 canal and the other organs, thus suspending them all from 

 the dorsal wall. Note in the anterior end of the peritoneal 

 cavity a large bi-lobed gland, the liver, red in fresh, 

 yellowish in alcoholic specimens. Its function, like that 

 of the liver of the toad, is to store up nutriment for the 

 blood and to secrete a digestive fluid called bile. Behind 

 the liver note a long, convoluted tube. What is this tube ? 

 Unfold this tube, separating it from its enveloping mem- 

 brane, the mesentery. Thrust a probe down the throat 

 and note that it passes into a thick-walled sac, the 

 stomach. The mouth and gill-slits open into the front 

 part of the alimentary canal called the pharynx, which 

 leads by a short tube, the cesopJiagus, into the stomach. 

 Note the large, thickened portion of the alimentary canal 

 leading from the stomach. This is the pylorus, and to its 

 walls are attached a number of finger-like projections, the 

 pyloric cceca. The pyloric caeca secrete a fluid which is 

 poured into the alimentary canal and which assists in the 

 process of digestion somewhat as does the secretion from 

 the pancreas of the toad. From the pylorus, passing 

 backwards in one or two loops, is the small intestine. 

 Trace this to its exit. Lying within the mesentery near 



