ALIMENTARY CANAL. 



123 



(beak), and certain toothless Whales bear horny plates (the so-called 

 whalebone) on their palate. 



In almost all cases the alimentary canal is provided in its different 

 regions with independent glands which mix their secretion with its 

 contents. In the cavity of the mouth the saliva secreted by a greater 

 or less number of salivary glands is mingled with the food. In many 

 aquatic animals these salivary glands may be reduced or be wholly 

 absent. Into the first part of the small intestine the bile and the 

 secretion of the pancreas, which is of great importance for the 

 digestion of the food, is poured. The bile is secreted by the liver 

 a large gland through which the venous blood returning from the 

 viscera passes -, a 



on its course 

 to the heart 

 (portal circu- 

 lation). In 

 Amphioxus the 

 liver is repre- 

 sented by a 

 simple csecal di- 

 verticulum of 

 the intestine. 

 In Amphioxus 

 and some other 

 fishes the pan- 

 creas is want- 

 ing. The small 

 intestine i n 

 which the 



juices are absorbed is distinguished not only by its great length 

 it is in fact this portion of the alimentary canal which is arranged 

 in coils but also by the presence of internal folds and papillae 

 which considerably increase the extent of absorbing surface. The 

 terminal region (large intestine, rectum) of the digestive canal is 

 principally distinguished by its width and its powerful muscles. 



Special respiratory organs, as lungs or gills, are always present. 

 The gills usually consist of double rows of lancet-shaped lamellae, 

 which are arranged on the sides of the pharynx behind the mandibu- 

 lar arch, and except in the Cydostomes are borne by visceral arches. 

 Between these arches there are always narrower or wider slit-like 

 openings, which lead directly into the pharynx and allow the water 



FIG. 679. The development of the tooth in Triton (after O. Hertwig). 

 a, The first stages of the development of a tooth ; on the right hand 

 is the earliest rudiment, b, Later stage of development. DK, 

 papilla of the cutis which later becomes the dentine of the tooth 

 MS, enamel membrane (epithelial growth which forms the enamel) ; 

 P, dentine; S, enamel; Up, epithelium of the mouth. 



