474 FRESH-WATER BIOLOGY 



The vagina is usually short and more or less muscular, especially 

 near the vulva, where its wall is usually thicker. At the thickest 

 part it suddenly diminishes in massiveness, and in the case of the 

 double-ovaried species forks to form two short tubular branches 

 which join the uteri. The walls of these two short tubes, as well 

 as those of the part nearer the vulva, are supplied with encircling 

 muscle fibers which by their peristaltic action force the egg onward 

 and outward in the process of deposition. The vulva is a trans- 

 verse slit-like opening whose length varies up to about one-half the 

 width of the body. Muscular fibers radiate from its cuticular 

 margin to the ventral submedian parts of the body wall, and serve 

 by their contraction to open the orifice. 



The subspherical to elongate eggs are covered with cuticular 

 shells of varying thickness, usually smooth, but sometimes bearing 

 projections. In the greater number of fresh-water species the eggs 

 are deposited before segmentation begins, but in some genera fully 

 developed embryos are formed in the eggs before deposition. A 

 few species are viviparous. The period of gestation varies widely. 

 In some cases the formation of the embryo occurs within the 

 space of a few hours to a day or two, in other cases weeks are 

 necessary. 



The structure of the testes resembles that of the ovaries, but 

 the resulting sexual cells, the spermatozoa, are smaller. The pri- 

 mordial germ cells at the blind end of the testis multiply to form 

 the grandmother-cells of the spermatozoa, which grow to a con- 

 siderable size, so that it is usually easy to locate the part of the 

 testis where they are maturing, generally the middle or proximal 

 part. These grandmother-cells, or spermatocytes, have the num- 

 ber of chromosomes characteristic of the males of the species, and 

 they proceed to the formation of the spermatozoa by a process of 

 sudden double division of the chromosomes such that each sper- 

 matocyte gives rise in most of the known cases to four spermatozoa, 

 two with half the number of chromosomes characteristic of the 

 females and two with one less chromosome than this. All these 

 spermatozoa are supposed to be potent, but there is a dearth of 

 experimental evidence. 



The oocytes follow a similar course but only one of the last 



