72 ANIMAL LIFE 



posterior part of the intestine, called the rectum, leads to 

 the anal opening. There is some differentiation of the 

 inner surface of the canal. In the great group of mol- 

 lusks, of which the common fresh-water clam or mussel is 

 an example, the alimentary canal (Fig. 35) shows much 

 variation. The microscopic plants, which are the food of 

 the mussel, are taken in through the mouth and pass into 

 a short oesophagus, thence into a wide stomach and there 

 digested. Behind the stomach is a long, much-folded, nar- 

 row intestine which winds about through the fleshy " foot " 

 and finally reaches the surface of the body, and has an 

 anal opening at a point opposite the position of the mouth. 

 Among the insects there is a great range in degree of 

 complexity of the alimentary canal. The digestive organs 

 are, however, in most insects in a condition of high speciali- 

 zation. The mouth opening is provided with well-developed 



Cll C 



PIG. 35. Pond mussel dissected to show alimentary canal, al. c. After HATSHEK 



and CORI. 



biting and masticating or piercing and sucking mouth parts ; 

 pharynx, oesophagus, stomach, and intestine are always dif- 

 ferentiated and sometimes greatly modified. In the com- 

 mon cockroach, for example (Fig. 36), the mouth has a 

 complicated food-getting apparatus, and the canal, which 



