132 



THE ANIMAL PARASITES OF MAN 



These two explanations are probably correct as far as each goes, and 

 are to be regarded as supplementary to one another. 



Occasionally, temporary associations of gregarines are formed 

 by a number of individuals adhering to 

 one another end to end. Such temporary 

 associations are examples of syzygy. Such 

 syzygies must not be confused with true 

 associations which form an essential part of 

 the life-cycle. 



The life-cycle of a relatively simple gre- 

 garine, such as Monocystis agilis (fig. 59), 



a b c parasitic in earthworms, may now be con- 



FIG. ^.Monocystis agilis. sidered. The gregarines, being members of 

 Spores from vesicula seminalis the Sporozoa, produce spores at one phase 



of the Earthworm, a, Sporoblast r . rr , 



with single nucleus, enclosed in or the lite-cycle. Each greganne spore 

 sporocyst ; 6, mature spore con- (fig. 6 3 ) develops within itself a number of 



taming sporozoites ; c, diagram- . -111 



matic cross-section of spore, minute, sickle-shaped or vermicular bodies, 

 showing eight sporozoites round known as sporozoites or primary infecting 



residual protoplasm. (After \ 



Butschli.) germs. Eight sporozoites are often formed 



within each spore. When absorbed by a new 



host, the spore softens and the sporozoites issue from it. They are 

 capable of active movement and may or may not enter a cell, such as 

 one of those of the digestive tract, or, as in Monocystis, a cell lining the 

 vesicula seminalis which becomes a sperm-cell aggregate (sperm morula). 

 When the sporozoite has reached the place of its choice in the host 

 it ceases active movements and proceeds to feed passively on the fluid 

 substances around it, whether they be those of tissues or body fluids. 

 This passive, growing and feeding form is known as the trophozoite. 

 After a trophic existence of longer or shorter duration, the trophozoite 

 ceases to feed and prepares for reproduction. Two trophozoites 

 associate together, each of them first becoming somewhat rounded. 

 The two trophozoites, now known as sporonts or gametocytes, become 

 invested in a single common envelope or cyst (fig. 64, a). The nucleus 

 of each gametocyte then divides by a series of binary fissions (fig. 64, b), 

 and the daughter nuclei thus produced arrange themselves at the 

 periphery of the parent cells (fig. 64, c). Cytoplasm collects around 

 each of these nuclei, and thus a number of gametes are formed within 

 each gametocyte. The gametes for a time exhibit active movements, 

 and ultimately ripe gametes of different parentage fuse in pairs, that 

 is, conjugation occurs (fig. 64, d). In this way zygotes are produced, 

 the nucleus of each zygote being formed by the fusion of two gamete 

 nuclei. 



The zygote grows slightly and becomes oval or elongate, and at 

 this 'period is often called the sporoblast. It then secretes an external 



