4 THE ESSEX NATURALIST.
animals with practically perfect external bilateral symmetry
are usually more or less asymmetrical, either in structure or in
function or in both; as, for instance, in all female birds in
which only the left ovary is properly developed, and in fishes
with homocercal tails which, while outwardly symmetrical, are
based upon an internal asymmetrical arrangement of the terminal
vertebrae. Naturally all the more striking cases of asymmetry
mentioned above are heritable as they are characteristic of
species or larger groups, and there are good grounds for believing
that even the smaller types of asymmetry are also in many
cases heritable. This is a very important fact, and one which
opens a point of attack upon the general problem of heredity.
Various degrees of asymmetry are equally well illustrated by
the Entomostraca as by the more obvious animals just referred
to. Being Crustacea and therefore belonging to the great
group of the Arthropoda, they are, like the vast majority of their
allies, typically bilaterally symmetrical. But there are many
cases in which more or less asymmetry occurs, sometimes only
in an occasional specimen, sometimes in certain species or genera
and families, and in one case throughout nearly a whole sub-class.
Thus, in the Branchiopodan order Cladocera there is scarcely
ever any departure from strictly bilateral symmetry either
internally or externally. Occasionally a specimen may show
a slight difference in the number of spines, etc, on the right
and left sides of the shell, post-abdomen, etc., but not as a
rule more than can be seen in different individuals of the same
species. The most extreme examples of asymmetry so far re-
corded in this Order are those of androgynous individuals, or
gynandromorphs as they are now usually called, of Daphnia
pulex and a few other species, which exhibit male characters on
one side and female characters on the other, in more or less
complete development. Such cases are no doubt rare, and I
have never yet met with one, but this type of asymmetry is of
a very interesting character, as it evidently has a most important
bearing on the vexed question of the determination of sex.
In contrast to the Cladocera, the Ostracoda are character-
ised by almost constant asymmetry. The two valves of the
shell in which these animals are enclosed are nearly always
slightly different in size and outline, corresponding to the fact
that one usually overlaps the other to some extent, or they may