158 COMPARATIVE ANATOMY 



pseudonavicella ; that some of them make their way into the, 

 sperm-mother cells of their host; whilst others are passed, 

 to the exterior by the seminal ducts, and remain alive for a, 

 while in damp earth. If they are then swallowed by another 

 earthworm, they pass through the walls of its gut into the 

 sperm-sacs, and find their way into the sperm-mother cells 

 of their new host. 



The spermatozoa of the earthworm are' formed by repeated 

 divisions of a sperm-mother cell ; but, instead of the whole of 

 the protoplasm going to form spermatozoa, a central cylinder 

 of residual protoplasm, the sporophore, is left, to which 

 the heads of the spermatozoa are attached. It is in this 

 sporophore that the sporozoite lives, and, absorbing its con- 

 tents, grows up into a mature Monocystis. The development 

 of the spermatozoa proceeds, in spite of the presence of the 

 parasite, and, eventually, the latter appears to be clothed with 

 a coat of long cilia, which are, in fact, the tails of the sper- 

 matozoa, now withering up for want of nutriment. Finally 

 the Monocystis, after absorbing all the available nourishment, 

 quits its cell-host, and is set free in the sperm-sac in a state 

 ready for conjugation. 



The chief interest of the Gregarine Monocystis lies in its 

 adaptation to a parasitic mode of existence. Relying entirely 

 for its nutriment upon the diffusible substances in the cell 

 (sporophore), in which it spends the greater part of its exist- 

 ence, its organisation is of the simplest possible character. 

 The absence of an aperture for ingestion of food, of pseudo- 

 podia, cilia, and contractile vacuole, point to a degradation of 

 organisation in connection with the parasitic habit. Still more 

 striking is the provision for perpetuation of the species by 

 means of an inordinately large number of young forms. A 

 large number of spores are formed in the first instance from 

 a pair of conjugating individuals, and, as each spore gives 

 rise to eight young forms, or sporozoites, the number of the 

 latter must be enormous. Though it is a matter of chance' 

 whether any one of them will find its way to a new host, or to 

 a new sperm-mother cell in the same host, the fecundity of the 

 zygote ensures that some few at least shall find their way to a 

 new cell-host, and survive. The sporozoites themselves repre- 

 sent the active phase of existence. Each is an actively mobile 

 animalcule, with a broader blunt extremity containing the 



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