I have never been able to convince
myself that the so-called "inner egg-membrane" is really of this nature,
and not perhaps the earliest larva skin, not formed until after
impregnation, as might be supposed with reference to Ligia, Cassidina
and Philoscia.) It will remind us of the union of the young Isopoda with
the larval membrane and of the unpaired "adherent organ" on the nape of
the Cladocera, which is remarkably developed in Evadne and persists
throughout life; but in Daphnia pulex, according to Leydig, although
present in the young animals, disappears without leaving a trace in the
adults.
The young animal, whilst still in the egg, acquires the full number of
its segments and limbs. In cases where segments are amalgamated
together, such as the last two segments of the thorax in Dulichia, the
last abdominal segments and the tail in Gammarus ambulans and Corophium
dentatum, n. sp., and the last abdominal segments and the tail in
Brachyscelus,* or where one or more segments are deficient, as in
Dulichia and the Caprellae, we find the same fusion and the same
deficiencies in young animals taken out of the brood-pouch of their
mother. (* According to Spence Bate, in Brachyscelus crusculum the fifth
abdominal segment is not amalgamated with the sixth (the tail) but with
the fourth, which I should be inclined to doubt, considering the close
agreement which this species otherwise shows with the two species that I
have investigated.
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