SEX DETERMINATION IN DINOPHIEUS GYROOILIATUS. 341 
that leads down a short penis, which, except during the act 
of copulation, is kept retracted within the penis sheath. 
I have been unable to distinguish whether the cilia of the 
anterior end are inserted in a well-marked groove as in D . 
conklinii. They are not so sharply limited to a ring or 
circle, but seem to cover almost the whole of the anterior end 
as shown in fig. 11. This appearance, however, varies very 
much with the state of contraction of the male, as in fig- 7 ; 
where the end is extended the cilia appear as a sharply limited 
circle. There is also this further difference between the males 
of D. gyu'ociliatus and both D. apatris and D. conklinii, 
in that the testis the vesicula seminalis is distinctly 
paired as shown in figs. 8, 11 ,and 14. This condition does 
not seem to hold in D. apatris, the males of which have 
been carefully figured by Korschelt (6) . 
If the young females that have just left the capsule are 
examined in the living condition, under an oil-immersion lens, 
it will be seen that the ciliated rings in the trunk, like 
those of the head reg’iou in the adult condition, are interrupted 
dorsally and are incomplete. The whole of the dorsal surface 
between the rings is also covered with fine very short cilia, 
and the female has a characteristic larval appearance. The 
relative size and configuration of the segments and general 
shape is also different from that of the adult condition. 
In the young female the head region is by far the widest part 
of the body, so that the animal seems to taper from the head 
towards the tail, and looks not unlike an attenuated wedge in 
shape, the thick end of the wedge being the head (Text-fig. 4). 
In the resting position shown in fig. 6 this shape does not 
show, but only appears during the act of swimming or moving 
rapidly. 
I first concluded that these females, of this peculiar shape, 
were the male forms of the species, for whenever I examined 
one of them under the microscope, 1 invariable found it to 
have sperm beneath the gut. I concluded therefore from this 
that I had found a new species of Dinophilus in which the 
females were much the same as inD. gy ro cilia t us, D. a pa- 
