96 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



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. anreus 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 



a 



a 



FIG. 41. Volvox globator. After Cohn. 

 a,, Balls of sperms ; b. t immature ova ; c. , ripe ova. 



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

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

 In r. globator their bundles may break up within the parent colony : 

 or, as always occurs in V. aureits, 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 



