122 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



aureus ; each contains a nucleus and a contractile vacuole. 

 At the anterior hyaline end, where the flagella are inserted, 

 there is a pigment spot ; the rest of the cell is green, owing 

 to the presence of chlorophyll corpuscles. In consequence 

 of the presence of these, Volvox is holophytic, i.e. it feeds 

 as a plant does and builds up starch granules. 



In its method of reproduction Volvox is of much biological interest 

 and importance. As Klein, one of its best describers, says, it is an 

 epitome of the evolution of sex. Some of the colonies are asexual. 

 In these a limited number of cells possess the power of dividing up to 

 form little clusters of cells ; these clusters escape from the envelope of 

 the parent colony, and form new free-swimming colonies. In other 

 colonies there are special reproductive cells, which may be called ova 

 and spermatozoa. 



In V. globator the two kinds of reproductive cells are usually formed 

 in the same colony, the formation of spermatozoa generally preceding 

 that of the ova. Technically the colony may then be described as a 

 protandrous hermaphrodite. 



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 possibiUties, 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. 



