SOME BIOLOGICAL PROBLEMS.
5
not be level when seen from the front, one being higher than
the other, producing a lopsided appearance.
The Copepoda, on the whole, are very symmetrical, but
some show extremely remarkable forms of asymmetry. For
example there is a species of Calocalanus (C. plumulosus) in
which one of the furcal setae on one side only is enlarged to such
an extent that it overshadows the whole of the rest of the animal,
being some five times as long as the body. But it is in the
family Centropagidae, which includes the genera Diaptomus,
Eurytemora, Heterocope, &c., and closely allied forms, that
asymmetrical development among the Entomostraca reaches its
culminating point, and strangely enough the phenomenon is
confined to the male sex. In these forms the right (in one
or two marine genera the left), antennule (first antenna) of the
male is modified into a clasping organ whereas the other anten-
nule is practically as in the female. Complicated modifications
of the right fifth foot and also in some cases of the abdominal
segments of the body on the right side also accompany the alter-
ation of the right antennule. With the exception of some
modification of the left fifth foot, these males are in fact, so far
as external characters go, practically female on the left side and
male only on the right side. How can such a state of affairs
have arisen ? Can it be that these males started as gynandro-
morphic sports similar to those which have been already alluded
to as occasionally occurring among the Cladocera ? The fact
that internally they are completely male does not preclude
this possibility, as gynandromorphs do occur in which the
combination of the male and female characters is confined to
the external organs. It may be held that a more serious argu-
ment against the suggestion is the fact that a whole series of
gradations exists among allied forms of Copepods, from species
in which the male practically only differs from the female ex-
ternally in a slightly different development on one side of the
fifth pair of feet, to the most pronounced type of asymmetry
referred to above. But this does not necessarily invalidate
the idea, as gynandromorphs of many different degrees have been
recorded and there seems no reason why the development of
asymmetrical form should not have proceeded progressively.
But in what way asymmetrical males could have had any super-
iority over symmetrical ones, as I think we must assume to