342 
CRESSWELL SHEARER. 
tris and D. conklin ii, while the males were simply much 
reduced in size and not rudimentary (without mouth, stomach 
and gut), as in the species just mentioned. I was confirmed 
in this supposition by the fact that these males in some jars 
never changed or grew any bigger, and as they contained 
active live sperm, they must therefore, I thought, be full grown 
adults. Now the fact that the females may remain indefi- 
nitely in the immature larval condition, to which I have already 
called attention, seems to depend on certain factors in the 
environment of which the presence of food only forms one. 
It so happened in the two jars from which I started all my 
subsequent cultures, one at the end of six months was swarm- 
ing with large females, while the other was full of what I 
thought at the time were the males. In this jar not a single 
large female was to be seen, nor did any appear after several 
mouths, although I supplied these small forms with abundant 
food. I was able, however, to demonstrate later that they were 
immature females, by taking a few of them and putting them 
in another dish, in which the conditions were somewhat diffe- 
rent, for they then grew rapidly into the large females. I was 
forced to conclude, therefore, that these small forms were not 
the males, and obtaining some eggs about the same time, I 
saw, as these hatched, the real males. 
At Plymouth D. gyrociliatus is never found in the rocky 
tidal pools in company with the red species D. teen i at us, and 
my material was first obtained from some sandy dredgings 
from Cawsand Bay, one of the smaller divisions of Plymouth 
Sound. This material had been allowed to stand in the labo- 
ratory for some months before it was noticed that it was 
swarming with D. gyrociliatus. I introduced it into one 
of the large laboratory tanks from this jar. There it soon 
established itself and has bred for the last three years. In 
this tank it has never become very abundant, although in 
smaller culture jars it sometimes increases in great numbers, 
without any very obvious reason. I have been unable to find 
out to what condition this sometimes sudden increase is to be 
attributed. Its food consists of various small algae, Pleuro- 
