156 THE SPOROZOA 



sacs. But all essential details of the life-history are quite similar in the 

 two species. 



The earliest known stage of Monocystis agilis is a minute 

 protoplasmic body, with a distinct nucleus, lodged in one of 

 the " sperm-morulae " floating in the sperm-sac. As is well known, 

 each sperm-morula of the earthworm gives rise to a cluster of 

 spermatozoa, attached by their heads to a central residual mass 

 of protoplasm termed the " sporophore." The young Monocystis 

 is found within the sporophore and grows at its expense and at 

 that of the attached spermatozoa. This stage of the parasite, 

 during which it is absorbing nutriment from its host and growing 

 rapidly, may be termed the trophic stage, and each individual 

 parasite during this stage may be termed a trophozoite. The 

 parasite soon becomes elongated in one direction. It assumes 

 first an oval contour and becomes later more or less vermiform. 

 As it grows it destroys the sperm-cluster in which it is lodged, 

 and in later stages it is found enveloped in an adventitious coat 

 or fur composed of the tails of the degenerated spermatozoa, 

 giving the appearance of a ciliated covering, which is thrown off 

 in the final stages of growth (Figs. 2, c, and 4, a). 



The full-grown trophozoite (Fig. 2) is still a single cell, with a 

 single nucleus. The body is limited by a distinct cuticle, within 

 which the protoplasm is differentiated into an external clear cortical 

 layer or ectoplasm, and an internal granular medullary layer or 

 endoplasm. The ectoplasm is the seat of contractility, and contains 

 in its deepest part a layer of fine contractile fibres, the so-called 

 myocyte-fibrillae. The endoplasm lodges the nucleus, and contains 

 numerous coarse granules representing nutriment held in reserve 

 for impending reproductive and developmental processes. The 

 nucleus is a clear spherical body, in the form of a vesicle 

 limited by a delicate membrane, containing fluid in which float 

 one or more nuclear corpuscles or karyosomes. Each karyosome is 

 a small globule, resembling in appearance the nucleolus of a 

 tissue-cell, but differing from it in containing a certain amount of 

 chromatin in its substance. The karyosomes usually have a 

 vacuolated structure. The trophozoite is actively motile, as the 

 specific name implies. In Monocystis the movements consist chiefly 

 of changes of form brought about by the contractility of the 

 myocyte-fibrillae, whereby the body may be bent or contracted 

 as a whole, or may exhibit ring-like constrictions in different 

 parts. 



After the trophic stage, which is a period of purely vegetative 

 growth, the parasite enters upon the reproductive phase of its 

 life-history, a period in which two distinct events follow each 

 other ; first, the formation of gametes or conjugating individuals, 

 which pair with one another and unite to form zygotes ; secondly, 



