176 MR. LUBBOCK ON SOME OCEANIC ENTOMOSTRACA 
perhaps, therefore, immature. Some of them had four joints, instead of three, to the 
ahdomen : this is generally characteristic of the male sex ; yet these specimens agreed 
with the others in the form of the abdomen and in other respects, and only differed from 
the mature females in the form of the fifth pair of legs. We do not, however, yet know 
whether, as in some other animals, the characteristics of the male sex appear first when 
the animal is mature, or whether in the young male the ahdomen, antennae, and fifth 
pair of legs already resemble those of full-grown specimens. I am inclined to doubt 
whether it be advisable to retain Dana's genus Hemicalanus. The characters by which 
it is separated from Diaptomus are not, I think, of great importance, and are both some- 
what inconvenient— the difference of size in the fifth pair of legs in the female, from 
being applicable only to one sex, and the absence of the four small intermediate segments 
of the second pair of antennae, because the joints between these segments become fainter 
and fainter so gradually that in some cases it is difficult to say whether they are present 
or not. 
Mr. Darwin, in his admirable work 'On the Origin of Species' (p. 156), observes that 
secondary sexual characters are very variable, that " species of the same group differ from 
each other more widely in their secondary sexual characters than in other parts of their 
organization;" and again, "that the secondary sexual differences between the two sexes 
of the same species are generally displayed in the very same parts of the organization in 
which the different species of the same genus differ from each other." The Entomostraca, 
and especially the Cyclopoidea, present remarkable examples of this law. In Fontella, 
for instance, the sexual characters are afforded mainly by the anterior antennae and the 
fifth pair of legs. The specific differences also are principally given by these organs ; and 
many of the generic characters in the Cyclopoidea are taken from the same source. 
The genera Calanus, Fontella, Euchceta, and others are very similar in form, live 
together in the open sea, and probably upon nearly the same food, and might, at first 
sight, be supposed to have similar habits. A glance, however, at the great differences in 
many of their appendages shows thaAhis cannot be the case, and proves to us how little 
we really understand of their habits and mode of life. 
CALANID^E. 
Calantjs. 
1. Seta antetmarum anticarum apieales subapicalibus longiores. Styli caudales vix 
oblongi. 
Calaxtjs latus, Lbk. 
Collected May 3, in & lat. 0° 40 7 , W. long. 0° 20'. 
2. Seta antennarum anticarum apieales subapicalibus breviores. 
A. Setce caudales ntediocres. 
Cephalothorax SS-articulatus, postice obtusus aut breviter subacutus. 
Calanus settjligerus, Dana. 
My specimens differed from those described by Prof. Dana in having the cephalothorax 
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