vii. 8 ALIMENTARY CANAL 201 



Above these a further series of dermal bones, the supra-clavicle and 

 post-temporal, attach the pectoral girdle to the otic region of the skull. 

 The pelvic girdle is very simple, consisting only of a single bone, 

 the basipterygium. 



6. Alimentary canal 



The food of the trout consists mainly of small invertebrates such as 

 Gammarus, Cyclops, and other crustaceans, and aquatic insects and 

 their larvae, together with the fry of other fishes and perhaps some- 

 times larger pieces of 'meat'. The food is mostly swallowed whole, 

 being helped down the pharynx by the mucous secretions, but these, 

 as in elasmobranchs, contain no enzymes. The entrance to the stomach 

 is guarded by a powerful oesophageal sphincter, no doubt serving to 

 prevent the entry of the water of the respiratory stream. The stomach 

 is divided into cardiac and pyloric portions, though the distinction is 

 less clear than in elasmobranchs. The duodenum is beset by a number 

 of wide-mouthed pyloric caeca, serving to increase the intestinal 

 surface (Fig. 120). The intestine and caeca are lined throughout by a 

 simple columnar epithelium and there are no specialized multicellular 

 glands such as the Brunners glands or crypts of Lieberkiihn of mam- 

 mals. The exocrine pancreas consists of numerous diffuse glands in 

 the mesentery. The endocrine portion, however, forms a compact 

 mass of tissue. This is very rich in insulin and after its removal a fish 

 shows hyperglycemia and glycosuria. The intestine is relatively longer 

 than in elasmobranchs and often coiled; its internal surface may be 

 increased by folds, but there is no true spiral valve, though this was 

 present in the ancestors of the Teleostei (p. 233). There is no gland 

 attached to the rectum. 



7. Air-bladder 



Dorsal to the gut is a very large sac with shiny, whitish walls, the 

 air-bladder, filled with oxygen. A narrow pneumatic duct connects 

 this with the pharynx in the more primitive forms. The origin and 

 functions of the air-bladder will be discussed below (p. 261); it serves 

 as a hydrostatic organ, enabling the animal to remain suspended in 

 the water at any depth. 



8. Circulatory system 



The general plan of the circulation is similar to that of an elasmo- 

 branch (Fig. 122), that is to say, there is a single circuit and all 

 the blood passes through at least two sets of capillaries. The heart 



