SEX DETERMINATION IN DINOPHILUS GYROCILIATUS. 349 
the egg-capsale as the female passes to the exterior. It is, 
however, possible for the same male to attach itself to the 
next female and so in turn fertilise her. In this manner I 
have seen a male fertilise three successive females as they 
were leaving the capsule. In the case of the third, it was 
carried some little distance outside before it finally succeeded 
in accomplishing this ; it then detached itself and slowly 
returned to the capsule, adhering motionless to it until it died 
in a few days. 
It is remarkable that the males are perfectly quiet within 
the capsule until the females start moving. Then they seem 
suddenly thrown into a state of great excitement and move 
about restlessly until they come in contact with the females. 
Immediately fertilisation is accomplished they become quiet 
Text-fig. 4. 
Young fertilised female, showing the outline and shape during 
swimming. 
again and remain motionless. Under the microscope, if the 
capsule happens to be compressed by the cover-glass and the 
normal conditions thus disturbed, the male seems occasionally 
to have some difficulty in forcing its penis through the body- 
wall of the female, and in so doing the sperms are sometimes 
discharged into the water. 
The ovary in general structure and position resembles that 
of D. conklinii and D. apatris. It occupies the concavity 
on the ventral side of the alimentary canal formed by the 
junction of the stomach and intestine. It is usually crowded 
with egg-cells, which push the intestine to one side. The 
ovary consists essentially of three parts, the obgonia, the 
oocytes, and the peritoneal sac enveloping the whole. The 
oogonia form a mass of closely packed round cells in the 
anterior region and send two lateral horns towards the head. 
The oogonial cells graduate in size as they proceed back- 
