The above operation will have severed the large powerful _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 membrane, 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 _œsophagus_, 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 cæca_. The pyloric cæca 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 the posterior end of the body-cavity note a small red glandular mass, the _spleen_.