ve) 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 cesophagus, thence into a wide stomach and there 
digested. Behind the stomach is a long, much-folded, nar- 
_ Tow 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 
al ¢: 
Fic. 35.—Pond mussel dissected to show alimentary canal, a/. c.—After HaTscHEK 
and Cori. 
biting and masticating or piercing and sucking mouth parts ; 
pharynx, esophagus, stomach, and intestine are always dif- 
ferentiated and sometimes greatly modified. In the com- 
mon cockroach, for exemple (Fig. 36), the mouth has a 
complicated food-getting apparatus, and the canal, which 
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