ALLIES OF THE EARTHWORM 



227 



right and left branch which open into sacs (Fig. Ill, 3). Food 

 taken into these sacs passes by osmosis into the tissues. The 

 granular masses (Fig. Ill, 4) are the excretory organs. In this 

 stage the ovaries and spermaries are not developed ; these 

 organs occur together in each adult. The body is not divided 

 into somites. 



The life-history of these parasites is usually very com- 

 plicated. Professor H. L. Jameson believes that Distomum 

 somaterise begins its life in a clam 

 found on the coast of France. At a 

 certain stage the young leave the 

 clam and make their way into the 

 mussel, Mytilus edulis (see Fig. 126). 

 The mussels are eaten by the black 

 scoter, a sea-duck, and in the body 

 of the bird the eggs of the parasite 

 are produced and sent out. By some 

 chance the larvse enter the clam, 

 and thus complete the cycle of devel- 

 opment. 



The class to which this parasite be- 

 longs is called Tremato'da (Gr. trema, 

 hole ; eidos, form, referring to appear- 

 ance of suckers). 



The Tapeworm. The best-known 

 tapeworm is the one that is sometimes 

 found to inhabit the intestine of man, 

 Tce'nia sagina'ta (Fig. 112). This parasite frequently grows to 

 the length of many feet. The figure seems to show that the 

 body is divided into somites, but these divisions are not con- 

 sidered true somites. The head of the animal is, except for 

 the narrow neck behind it, the smallest portion of the body ; 

 the posterior portion is the oldest part of the parasite, except 

 the head itself. New somite-like divisions of the body form 



FIG. 111. Treinatode. 

 Much enlarged. (After 

 Jameson) 



1, anterior sucking-disk, 

 and mouth ; 2, pharynx ; 

 3, digestive sac; 4, excre- 

 tory organs ; 5, posterior 

 sucking-disk 



