102 



PRINCIPLES OF ANIMAL BIOLOGY 



MOUTH-*^"^PHARYNX 



Fig. 89. — Digestive system 

 (coelenteron) of a rhabdocoele 

 flatworm (above) and a triclad 

 turbellarian. 



it is a result of degeneracy. Other flatworms have a three-branched 

 coelenteron, each part of which is extensively branched (Fig. 89, below). 

 As animals rise in the scale of complexity the digestive system becomes 

 a tube open at both ends. One end is the mouth, which ingests food, 

 the other end the anus through which undigested, mostly indigestible, 



matter is ejected. In the course of the 

 tube it is differentiated into organs. In 

 the earthworm (Fig. 88), following the 

 mouth, there is a short buccal cavity, a 

 -pharynx with strong muscular walls, an 

 esophagus^ a croy in which food may be 

 stored, a gizzard with thick muscular 

 walls and a chitinous lining by means of 

 which food may be finelj^ ground, and an 

 intestine with secreting and absorptive 

 cells. An internal ridge, the typhlosole, 

 formed by an infolding of the dorsal wall 

 of the intestine (Fig. 66), 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 diges- 

 tive system 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. A diagram repre- 

 senting vertebrates in general fairly well, but more particularly the 

 mammals, is at the bottom of Fig. 88. The system in the frog is slightly 

 more simple (Fig. 90, left). In the mouth 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 cavity or 

 mouth cavity leads backward into the short broad esophagus through a 

 distensible 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 middle line. The walls of 

 both the esophagus and stomach are provided with highly developed 

 glands which secrete digestive solutions. The stomach opens into the 

 small intestine through a muscle-encircled passage, the pylorus. The 

 small intestine of vertebrates is usuall}^ subdivided into three portions 

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

 duodenum and ileum alone are recognized in the frog. These regions 

 as a rule merge imperceptibly into one another, yet each shows certain 



