MONOCYSTIS 



Among the organs of reproduction of an earthworm are certain 

 sacs, known as the seminal vesicles, in which the sperms ripen. 

 Here are generally to be found specimens of the parasites known 

 as MonocysHs (Fig. 17), which live by absorbing, through the 

 surface of their body, the fluid in the vesicles which is intended 

 for the nourishment of the spermatozoa. Two kinds of these 

 creatures may be present, differing in size 

 and in certain other particulars. The larger 

 kind, M. magna, is easily visible to the 

 naked eye as white threads, hanging by 

 one end from the funnels of the vasa 

 eferentia (see p. 175). The smaller, known 

 as M. lumbrici, is more often found 

 free in the fluid among the developing 

 spermatozoa. The body of a full-grown 

 MonocysHs is long and narrow, and consists 

 of a soft, granular endoplasm and a firm, 

 clear ectoplasm. The endoplasm contains 

 numerous granules, many of which consist of 

 the carbohydrate substance paraglycogen, 

 and the ectoplasm is covered with a stout 

 cuticle and has in its deeper layer a net- 

 work of contractile threads, the myonemes. 

 While the cuticle makes it impossible for 

 the protoplasm to flow out into pseudo- 

 podia, the myonemes enable the animal 

 to change its shape by squeezing the fluid 

 endoplasm from one part of the body to 

 another. Slow waves of contraction of this 

 kind are constantly passing along the 

 body. In the endoplasm there is a large 

 nucleus, but there is no contractile vacuole. 

 At one end of the body an indefinite knob 

 enables it to adhere to one of the cells of 

 the funnel. 



Fig. 17. — MonocysHs. 



A, M. tnagfhi ; B, M. lumbnci. 

 The latter is covered with 

 the tails of spermatozoa, the 

 offspring of the sperm mother 

 cell in which it was embedded. 



4? 



