FUNCTION AND STRUCTURE 



69 



tact with the food particle, and the body substance simply 

 incloses the food (Fig. 3). Food is taken in by the sur- 

 face. The whole outer surface of the body is the food- 

 taking organ. In the simplest many-celled animals, the 

 sponges, there is no special food-taking and digestive organ. 

 Each of the cells of the body takes in and assimilates food 

 for itself. The sponge is like a great group of Amoeba 

 holding fast to each other, but each looking out for its own 

 necessities. Among the ^ ? 



polyps, however, there 

 is a definite organ of 

 digestion that is, food 

 is only taken and di- 

 gested by certain parts 

 of the body. The sim- 

 ple polyp's body (Fig. 

 31) is a cylinder or vase 

 closed at one end and 

 open at the other end, 

 and attached by the 

 closed end to a rock. 

 The opening is usually 

 of less diameter than 

 the diameter of the 

 body, and it is sur- 

 rounded by a number 

 of tentacles, whose 

 function it is to seize the food and convey it to the mouth 

 opening. There are, of course, no teeth, no tongue, none 

 of the various parts which are in or are part of the mouth 

 of the higher animals. The polyp's mouth is simply a 

 hole or opening into the inside of the body. This body 

 cavity, or simplest of all stomachs, is simply the cylindrical 

 or vase-shaped hollow space inclosed by the body wall. 

 This space extends also into the tentacles. There is no 

 other opening, no posterior or anal opening. We can not 



FIG. 31. Obelia sp., a simple polyp; vertical sec- 

 tion, highly magnified, m, month opening ; 

 al. s., alimentary sac. After PAKKER and 

 HASWELL. 



