1912.] R. B. SEYMOUR-SEWELL: Surface-Living Copepoda. 343 
L. euchaeta. Stage 4. 
Pl. xix, figs. 8 and 9g. 
This is the earliest stage in which any definite difference be 
tween the two sexes could be made out, and here the only appa- 
rent sexual indication lay in the proportional length of the 5th 
legs; in the female they were short, reaching only to a point half- 
way or a trifle more along the 2nd abdominal segment, whereas 
in the male they reach well beyond the 2nd segment and in some 
cases half-way along the 3rd. 
In both sexes the form of the legs were the same and con- 
sisted of two joints, the distal having only three spines, and in 
both sexes also the abdomen consists of three segments, of which 
the third is the longest. 
From the above descriptions, it is seen that the two final 
stages (I and 2) differ very considerably as regards certain 
characters; especially is this the case in the males, where the 
grasping antennae and the 5th pair of legs are entirely differ- 
ent. 
In the females the differences are less marked, yet a study of 
the two sexes in these stages must, I think, convince anyone that 
they actually belong to each other and that we are not dealing 
with the female of one species and the male of another. For the 
purpose of reference I have tabulated below some of the main 
characters following the changes through the four final growth- 
moults. 
| si 
| | | 
|No. of joints in y | sth les 2 
Peepdaaien Head lenses. | ist antenna ¢. | exopod. 
| | 
| ms 2 
$ g | 
Stage 4 3 3 | No lenses .. | 23 free segments | 3 spines, 
Saad 3 Awe Norensessaa 23 free segments | 4 spines. 
Smee a2 3 5 | No rostral len- | 23 free segments | 4 spines. 
ses, small ven- 
tral lens. | 
ae! 56 2 5 No rostral len-| 21 joints, seg- | 5 spines. 
ses, ventral} ments 7 and 8, 
lens. and Io and 11 | 
fused. 
| 
It is at once obvious that these differences are due to a pro- 
gressive development through the various growth-moults. In all 
the above-mentioned instances there is a regular gradation such 
as could not be merely due to coincidence. There is no greater 
difference between stages 1 and 2 than there is between stage 2 
and the sexually immature forms, stages 3 and 4, and I consider 
that this fact, combined with a study of the developmental 
moults, justifies me in concluding that these are dimorphic forms 
