PISCES. 161 



creatic duct is a short tube carrying off the pancreatic secretion 

 and opening into the left side of the intestine about the same 

 level as the bile-duct. 



A short tube with thickened walls, the rectal gland, opens 

 into the dorsal side of the rectum. 



The abdominal cavity, in which most of the digestive organs 

 are contained, is lined by a thin membrane, the peritoneum, 

 which leaves the body- wall in the median dorsal line to form 

 a double sheet, the mesentery, the halves of which diverge and 

 wrap round the gut, liver, &c., constituting suspensory folds, 

 which, however, are for the most part very incomplete. 



The dogfish is a very voracious animal, feeding upon other 

 fishes, Crustacea, and molluscs. In some districts, at any rate, 

 it is especially abundant during the herring season. The rows 

 of sharp backwardly-pointed teeth assist in securing the prey. 

 By means of contractions of the muscular walls of the gut the 

 food is gradually passed backwards, and the force expended 

 during this process, combined with the softening and chemical 

 action of the digestive juices, serves to disintegrate it. The 

 chief digestive juices are the gastric juice, pancreatic juice, and 

 bile, of which the first is secreted by small glands in the wall 

 of the stomach and contains a ferment which converts proteids 

 into soluble diffusible peptones. The pancreatic juice, also 

 by ferment action, completes the digestion of proteids, converts 

 starch into sugar, and emulsifies fats. Bile assists in the last 

 kind of digestion. 



The digested food diffuses into the blood-vessels and lym- 

 phatics which ramify in the wall of the gut. An increased 

 absorptive surface is given by the spiral valve, which also 

 prevents the contents of the intestine from passing backwards 

 too rapidly. The comparative shortness of the gut is correlated 

 with the easily digestible animal diet. 



5. The circulatory organs of the dogfish comprise (I.) a blood 

 system, and (II.) a lymphatic system. 



(I.) The "blood system (Fig. 46) is a closed set of tubes con- 

 taining red blood, consisting of coagulable plasma, in which 

 colourless corpuscles and red corpuscles are suspended. The former 

 are amoeboid and nucleated, the latter are oval discs, containing 

 a well-marked nucleus, and coloured red by hemoglobin. 



A heart, arteries, veins, and capillaries can be distinguished. 

 2 11 



