ON THE NEMATODES OP THE COMMON EAETHWORM, 635 
sperm, the other half giving' rise to eggs only. The males 
also occasionally “ re-fertilise ” hermaphrodites whose stock 
of spermatozoa has become exhausted. In the present species, 
however, I did not see any of these partially hermaphrodite 
individuals or discover any cases of re-fertilisation. 
It may be asked why bisexuality does not, like hermaphro- 
ditism, admit of experimental proof, without actual obser- 
vation of the males in copulation with the females. The 
reason is that the results of an experiment may be misleading. 
A female placed with a male may reproduce, whilst a female, 
isolated when larval as a control, may not. This result might 
at first be regai'ded as an almost undoubted case of reproduc- 
tion. But the real facts may be that the female placed with 
the male reproduced as a hermaphrodite, developing sperma- 
tozoa for itself and not receiving them from the male, in spite 
of appearances to the contrary, while the isolated female 
refused to reproduce, not because of the absence of males, 
but from some other cause. It should be mentioned that in 
this species, on account of the position of the gut, it is often 
vei’y difficult, especially in young or unproductive females, to 
distinguish whether spermatozoa are present in the reproduc- 
tive organs or not. 
(2) The Sex Ratio. 
In regard to the sex ratio the results of the foregoing 
cultures and other observations not recorded here work out 
as follows. The nematodes removed from the freshly killed 
worm and reai-ed in cultures, or obtained from the dead worm 
decaying under natural conditions, develop into males and 
females. Subsequent generations, however, bred under cul- 
tural conditions consist in almost all cases of females only. 
This difference between the ratio of males to females in the 
parental and in the filial generations under cultural conditions 
is most striking. In the parental generation the ratio of males 
to females was about 1 to 5, which for a hermaphrodite species 
is very high. After this the ratio was, with few exceptions. 
