download pdf

Tribe 6.
EPICARIDA.
The Isopoda belonging to this tribe are all parasitic forms, and their
hosts are invariably other Crustacea of different orders ; hence the above name
of the tribe. They are the most degradated forms of Isopoda, and in some instances
the fully-grown female represents merely a simple sac rilled with ova, and
without any trace of segmentation or of limbs, so that the idea of its being
a crustacean animal would hardly suggest itself, if the development were not
known. In all the forms a more or less pronounced regressive metamorphosis
has been observed, comprising at least two distinct larval stages, very dis-
similar both in their general appearance and in the structure of the
several appendages, and equally different from the adult animal. As will
be shown further below, there is in all probability in most of the forms even a
3rd intermediate larval stage, the Microniscus stage, which, however, has not
hitherto been recognized as such, but has been wrongly regarded as an adult form
of Epicarida representing even a distinct family. The sexual dimorphism in
all the forms is very pronounced. The male in all cases, as compared with
the adult female, is of diminutive size, and is generally found clinging to the
genital region of the female like a parasite. It is also of an appearance very
different from that of the female, retaining, as it does, several of the larval
characters, and in some instances it does not differ from the last larval stage
except by the presence of generative organs. In certain cases (Entoniscidre), the
parasite penetrates to the inner organs of its host, looking merely
like an
endoparasite, and in all such cases the body of the female is found to be greatly
deformed. But when the parasite retains its character as a true ectoparasite,
the female also exhibits, as a rule, some more or less pronounced deformity - - and
is often conspicuously asymmetrical; even when the segmentation of the body is