(FIGURE 38. Embryo of a Philoscia in the egg, magnified 25 diam.)
To the question, how far the development of Ligia is repeated in the
other Isopoda, I can only give an unsatisfactory answer. The curvature
of the embryo upwards instead of downwards was met with by me as well as
by Rathke in Idothea, and likewise in Cassidina, Philoscia, Tanais, and
the Bopyridae,--indeed, I failed to find it in none of the Isopoda
examined for this purpose. In Cassidina also the first larval skin
without appendages is easily detected; it is destitute of the long tail,
but is strongly bent in the egg, as in Ligia, and consequently cannot be
mistaken for an "inner egg-membrane." This, however, might happen in
Philoscia, in which the larval skin is closely applied to the
egg-membrane (Figure 38), and is only to be explained as the larval skin
by a reference to Ligia and Cassidina. The foliaceous appendage on the
back has long been known in the young of the common Water Slater
(Asellus).* (* Leydig has compared this foliaceous appendage of the
Water Slaters with the "green gland" or "shell-gland" of other
crustacea, assuming that the green gland has no efferent duct and
appealing to the fact that the two organs occur "in the same place."
This interpretation is by no means a happy one. In the first place we
may easily ascertain in Leucifer, as was also found to be the case by
Claus, that the "green gland" really opens at the end of the process
described by Milne-Edwards as a "tubercule auditif" and by Spence Bate
as an "olfactory denticle.
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