ioo PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



In V. aureus the colony is oftenest unisexual or dioecious, i.e. either 

 male or female. But it may be monoecious or hermaphrodite, and is 

 then generally protogynous, i.e. producing eggs first. 



Whether in a hermaphrodite or in a unisexual colony, the sex cells 

 appear among the ordinary vegetative units ; the ova are distinguishable 

 by their larger size, the " sperm mother cells" divide rapidly and form 

 numerous (32-100 or more) slender spermatozoa, each with two cilia. 

 In V. globator their bundles may break up within the parent colony ; 

 or, as always occurs in V. aureus , they may escape intact, and swim 

 about in the water. In any case, an ovum is fertilised by a spermato- 

 zoon, and, after a period of encystation and rest, segments to form a 

 new colony. Occasionally, however, this organism, so remarkable a 

 condensation of reproductive possibilities, may produce ova which 

 develop parthenogenetically. 



Here, then, we have an organism, on the border line between plant 

 and animal life, just across the line which separates the unicellular from 

 the multicellular, illustrating the beginning of that important distinc- 

 tion between somatic or body cells and reproductive cells, and occurring 

 in asexual, hermaphrodite, and unisexual phases. Klein records no less 

 than twenty-four different forms of V. aureus from the purely vegetative 

 and asexual to the parthenogenetic, for there may be almost entirely 

 male colonies, almost entirely female colonies, and other interesting 

 transitional stages. Klein has also succeeded to some extent in showing 

 that the occurrence of the various reproductive types depends on outside 

 influences. 



Seventh Type of Protozoa MONOCYSTIS 



MonocystiS) a type of Sporozoa in which the cell is not 

 divided into two parts by a partition. 



Description. Two species (M. agilis and M. magna) 

 infest the male reproductive organs of the earthworm. The 

 full-grown adults are visible to the naked eye, flattened 

 worm-like cells ; the shape alters during the sluggish move- 

 ments. Peripherally there is a porous cuticle, a clear cortical 

 zone, and a network of myoneme fibrils. In a more fluid 

 medullary substance, the large nucleus floats. There are 

 numerous granules of protein, carbohydrate, and other sub- 

 stances. In one species there is an anterior projection like 

 the cap of Gregarina^ otherwise unrepresented in Monocystis. 

 As in Gregarina, and many other parasitic forms, a con- 

 tractile vacuole is absent. 



Life history. The young form of M. agilis is parasitic 

 within one of the sperm mother cells of the earthworm. It 

 grows, and becomes free from the cell as a trophozoite. In 



