CHAPTER VIII 



MONOCYSTIS 



Among the organs of reproduction of an earthworm are 

 certain sacs, known as the vesiculae seminales, 

 Features. m which the sperm ripens. Here are generally 

 to be found specimens of the parasites known 

 asMonocystis, which live by absorbing, through the surface 

 of their body, the fluid in the vesiculag which is provided 

 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 deferentia (see p. 228). 

 The smaller, known as M. agilis, is more often found free 

 in the fluid among the developing spermatozoa. The body 

 of a full-grown Monocystis 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 starch-like substance paramylum, and the 

 ectoplasm is covered with a stout pellicle and has in its 

 deeper layer a network of contractile threads, the myonemes. 

 While the pellicle makes it impossible for the protoplasm to 

 flow out into pseudopodia, the myonemes enable it 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 con- 

 tractile vacuole. At one end of the body a small knob or 

 epimerite enables it to adhere to one of the cells of the 

 funnel. 



In the stage which we have just described, the animals 



