VERTEBRATA. 69 



imbedded in the substance of the liver, as in many of the inferior 

 mollusca. The mollusca are without gall-bladder^ and as yet no 

 portal circulation has appeared. The entrance of the hepatic duct 

 into the stomach is guarded by a pair of prominent valves, pro- 

 longed so into the intestine as to allow of the passage of the pan- 

 creatic and biliary secretions into the gut during a state of vacuity 

 without entering the stomach or duodenum. 



CHAPTER XI. 



VERTEBRATA. 



In this great department of the animal world the digestive appa- 

 ratus presents a higher development and a greater degree of com- 

 plexity than we have met with before. The alimentary canal 

 always swells out into a distinct gastric enlargement, and is provided 

 with a large conglomerate liver, spleen and pancreas. The duode- 

 num invariably receives the biliary and pancreatic secretions, and 

 he salivary glands are rarely absent. The teeth are generally 

 confined to the alveoli, and there are none found in the stomach. 

 As a general rule the canal is longer, larger and more sacculated, 

 and the masticating; and glandular apparatus more developed in 

 the phytophagous than in the carnivorous tribes. 



PISCES. 



Since vegetable food cannot be procured in the unfathomable 

 depths of the sea, we are prepared to meet with a short and simple 

 form of alimentary canal in fishes, suited to their predaceous habits. 

 Vegetable substances might even endanger their lives by an evolu- 

 tion of gas which would render them specifically lighter than the 

 water, and cause them to float upon its surface with the belly up- 

 wards. The teeth of fishes are generally numerous, destitute of 

 fangs, laminated, deciduous, thinly covered with enamel, and 

 movable on the surface to which they adhere, till maturity, when 

 they become permanently fixed. The teeth of fishes are often 

 implanted in the tongue as in many gasteropods and birds, in the 

 vomer as in amphibia, in the palate bones as in serpents, in the 

 pharyngeal bones, branchial arches and os hyoides, as well as in the 

 maxillary and inter-maxillary bones to which they are confined in 

 the saurian reptiles and mammalia; the teeth are most numerous 

 in the pike and salmon, and are altogether wanting in some genera, 

 as the sturgeon. As in serpents, crocodiles, dolphins, and other 

 predaceous non-masticating vertebrata, the teeth are not opposed to 

 each other but placed alternately, adapting them to their prehensile 

 office. 



