THE ALIMENTARY SYSTEM 345 



method, and the Gnathostomes were thereby able to evolve 

 to greater size. In these forms also, the jaws are garnished 

 with teeth, and the nature and shape of the teeth varies with 

 the kind of diet. Not only do teeth assist in seizing prey, 

 but in the higher forms they serve to grind it up small, which 

 is an aid to the processes of digestion. In the higher verte- 

 brates, the tongue may also be used for obtaining food as in 

 the case of the chamaeleon, and it assists in the process of 

 swallowing. 



In the primitive forms the alimentary canal or gut runs 

 straight from mouth to anus, as in Amphioxus and the Cyclo- 

 stomes. In these two forms the lining of the gut is ciliated, 

 but in higher forms the ciliation is restricted to certain anterior 

 regions. Food is propelled along by peristaltic action of the 

 smooth muscle in the gut- wall. 



Beginning in the Selachians, a special part of the gut is 

 modified as a receptacle in which food is treated with digestive 

 juices secreted by its walls, and in which absorption does not 

 take place. This is the stomach, and in all Gnathostomes it is 

 an enlarged region of the gut, kinked to the left side of the 

 body, and situated between the non-digestive supply-tube or 

 oesophagus and the absorbent intestine. The intestine of the 

 Gnathostomes is greater in length than the space which con- 

 tains it, with the result that it is more or less coiled. In the 

 higher forms the intestine is very considerably longer than the 

 body itself. The effect of this is to increase the surface of 

 absorption. A modification which serves the same function 

 is the spiral valve in the intestine, which is feebly developed 

 in Petromyzon and well developed in the Selachians. The 

 spiral valve is also present in Ceratodus and in a few primitive 

 bony fish (Teleostomes), but it is lost in all higher forms. A 

 peculiarity of the stomach of the higher bony fish (Teleostei) 

 is the development of a number of blind outgrowths (pyloric 

 coeca) from the hinder end of the stomach. The wall of the 

 intestine is well supplied with blood-vessels belonging to the 

 hepatic portal system, and with lymphatic vessels or " lacteals." 



The oesophagus in birds is modified and enlarged into a 

 crop or temporary storage place. The stomach is divided 



