472 FRESH-WATER BIOLOGY 



proaches maturity these sexual cells resume their activity and 

 begin to divide and to produce a symmetrically two-parted elon- 

 gated group of cells, one part extending forward and the other 

 backward. Primarily the sexual organs of both sexes are double, 

 and the normal development at first always forecasts a double 

 organ. This forecast is often fulfilled, but in many species one of 

 the halves has deteriorated or become vestigial. Where this is 

 the case the symmetry of the early development is soon lost and 

 the group of developing sexual cells then becomes one-sided. 



At the last moult, or the penultimate, the sexual opening in the 

 cuticula makes its appearance. This is always on the ventral side, 

 and in the male invariably corresponds with the anus; in the female 

 it is independent and nearer the middle of the body, usually very 

 near the middle when the internal organs are double and symmet- 

 rical, and farther back, or more rarely farther forward, when there 

 is only one ovary. 



The female sexual system is very commonly double, each half of it 

 being tubular and consisting of (i) an ovary, (2) a seminal receptacle, 

 (3) a uterus, (4) a vagina; this latter of course in common with the 

 other half of the apparatus. These parts may lie in linear succession 

 in the body cavity, or, as is more often the case, the series may be 

 folded near its middle, that is, between the ovary and the uterus, 

 so that the ovary is reflexed and extends back toward the vulva. 



The more usual forms of female apparatus are as follows: 



1. Of two parts, each reflexed. 



2. Of two parts, each outstretched. 



3. Single and reflexed. 



4. Single and outstretched. 



When the organ is single it may extend either forward or back- 

 ward from the vulva, though it usually extends forward. Letting 

 F represent the vulva, - an outstretched organ, and ' a reflexed 

 organ, the various forms may be abbreviated as follows: 



<p -F- 'F F' -F F- 



and this is the form in which the facts are presented in the measure- 

 ment formulae for the females, except that F is replaced by the per- 

 centage measurement figure representing the position of the vulva. 



