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CRESSWELL SHEARER. 
by Repiaclioff ( 13 ) that this species is identical with D. gyro- 
ciliatus, and its specific value seems somewhat doubtful. 
No one who has worked on D. gyrociliatus has ever 
worked on D. apatris, so it seems impossible to determine 
its real value. I have sent material of my species to Prof. 
Korschelt, who has kindly examined it for me, but lias been 
unable to report anything further than that it is undoubtedly 
closely related to D. apatris. In the last few years this 
species has been made the subject of considerable investi- 
gation by Yon Malsen ( 8 ) in Prof. Hertwig’s laboratory, and 
a special variety has been described under the name 
of “P. apatris forma t ergestina,” from Trieste, by 
Stiasny ( 18 ). The second species with which D. gyro 
ciliatus may be related is the American formD. conklinii. 
With this species the Plymouth variety has possibly less 
relationship than with D. apatris. Dr. Nelson ( 10 , 11 ), 
who has made D. conklinii the subject of two extensive 
memoirs, as well as Prof. Conklin ( 3 ), who has done con- 
siderable work on the oogenesis, have kindly examined 
material of the Plymouth species for me, and they both 
report that they consider it different from their species. 
M. de Beauchamp also tells me he thinks the Plymouth 
species different from the form he has investigated at Roscoff, 
which he considers the same as D. conklinii. 
The females of D. gyrociliatus at Plymouth are very 
worm-like in their general appearance (figs. 1, 2, and 3.) 
The body lengthens out very considerably in the usual swim- 
ming attitude, then measuring quite 1*5 mm. in length. 
While at rest it is more contracted and the segmentation 
more pronounced. The head is blunt, conical, and rounded 
in front, and narrows into a constricted neck region, where 
it joins the trunk. This region, however, in D. gyro- 
ciliatus is not so marked as in the allied species 
D. conklinii. The ventral surface of the head is very 
much flattened, while its dorsal surface is rounded, and bears 
a pair of red crescentric eyes. The two first trunk segments 
are much bigger than in D. conk 1 in i i, where they are some- 
