SEX DETERMINATION IN D1NOPHILUS GYROCILIATUS. 339 
what reduced ; the caudal appendage or last segment is 
sharply marked and is conical and more pointed. 
Like other members of the group, the female of D. gyro- 
ciliatus is encircled by a number of ciliated rings, nine in 
number, of which two that are interrupted dorsally belong' to 
the head, while seven belong to the trunk region. The 
ninth band is on the caudal appendage and is much reduced. 
In addition to the ciliated bands, the ventral surface is 
extensively ciliated in the pre-oral and oral region, and then 
running back along the entire ventral surface is an extensive 
tract that gradually narrows as it approaches the caudal 
segment, where it is much restricted. The ciliated bands are 
narrow and furnished with relatively few but very powerful 
cilia. Those of the trunk are narrow transverse bands, that 
are uninterrupted on the dorsal surface; each baud 
encircles the middle of the segment. The two bauds of 
the head, as in D. apatris and D. conklinii, are inter- 
rupted dorsally and relatively wider than those of the 
trunk. In addition to the ciliated tracts and bands, D. gyro- 
ciliatus possesses conspicuous sense-liairs, two of which are 
arranged symmetrically, on the anterior surface of the head, 
and are undoubtedly tactile in function. There are five pairs 
of solenocyte bearing nephridia, whose arrangement can be 
best seen in Text-fig. 2 and figs. 3 and 5. 
The males are small and very inconspicuous relatively to 
the females, and, as I have said, are seldom seen outside the 
egg-capsule. Korschelt (6) has also remarked on the fact 
that in D. apatris the males are rare and live only a few 
days, and Hallez (5) has mentioned that it is a singular fact 
that the males of Dinophilus are never found in company 
with the females. Their shape is subject to considerable 
variation dependent on their state of contraction. It roughly 
agrees with that of the males in 1). apatris and D. con- 
klinii. When moving rapidly their shape is much as that 
shown in fig. 10, while when at rest it is as in figs. 8 and 1 1. 
When moving about slowly within the egg-capsule it is more 
like that of figs. 7, 14, and 15. About the anterior end of the 
