MORPHOLOGY OF HIGHER TYPES 



117 



the esophagus, but there is no differentiation into stomach and intestine. 

 However, in one feature, the presence of two openings, there is an advance 

 over all the digestive systems previously described. 



The digestive system of the Annelida, of which the earthworm may be 

 taken as an example, shows considerable differentiation into organs. It 

 consists (Fig. 83) of a mouth, opening into a buccal cavity, a -pharynx with 

 strong muscular walls, an esophagus, a crop in which food may be stored, 

 a gizzard with thick muscular walls and a chitinous lining by means of 

 which food may be triturated, an intestine with secreting and absorptive 

 cells, and an anus. An internal ridge, the typhlosole, formed by an in- 

 folding of the wall of the intestine gives increased surface. About the 

 exterior surface of the intestine is a layer of 

 brown cells, the chloragogen cells, which have 

 been thought to serve as a digestive gland, 

 possibly as a liver. 



Digestive Systems in the Vertebrates. — In 

 the vertebrates the digestive sj^stem reaches 

 its highest development. Here it consists 

 not only of an alimentary canal, subdivided 

 into regions, but also of highly developed 

 glands which produce digestive secretions. 

 The brief description below is based on the 

 digestive system of the frog which is illus- 

 trated in Fig. 84. 



The mouth, although it functions also in 

 the respiratory processes, is to be regarded as 

 part of the digestive system. The form of 

 the mouth and mouth cavity is determined 

 largely by the skeletal features of the head. 

 The upper jaw bears teeth which serve to 

 hold the prey when caught. Attached to the 

 anterior portion of the floor of the mouth is 

 a prehensile tongue which is provided with 



many glands that produce a sticky secretion. The buccal or mouth 

 cavity leads backward into the short broad esophagus through a disten- 

 sible opening the pharynx. The esophagus leads into the muscular 

 stomach which in the frog as in most vertebrates is a curved organ 

 usually lying somewhat to one side of the body cavity. The walls of 

 both the esophagus and stomach are provided with highh^ developed 

 glands which secrete digestive solutions. The stomach opens into the 

 much coiled small intestine through a muscular valve, the pylorus. 

 The small intestine of vertebrates is usually subdivided into three 

 portions named, respectively, the duodenum, jejunum, and ileum. Of 

 these the duodenum and ileum alone are recognized in the frog. 



Fig. 81 . — Digestive and 

 nervous systems of a rhabdo- 

 coele turbellarian. g, gang- 

 lionic mass; int, intestine; n, 

 nerve; ph, pharynx. {After 

 Lang.) 



