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General Remarks,
The present order being rather nearly allied to that of the Amphipoda,
a similar terminology may on the whole be applied to both. The body, which in
most cases exhibits a more or less depressed, not as in the Amphipoda com-
pressed, form, admits of being divided into 3 chief sections, viz., cephalon, meso-
some and metasome, the urosonie not being, as in the Amphipoda,
defined as a
particular division. In the group CheUfem, as also in the family Gnathudas among
the FlabeUifera, the cephalon is coalesced with the 1st segment of the mesosome, for
which reason, in the said forms, this section may more properly be termed cephalosome.
As to the several appendages, those of the cephalon are the same as in the
Amphipoda, and are denominated in a similar manner. The 2 pairs of antennae,
it is true, are generally described as inner and outer, not as superior and inferior;
but on a closer examination it may be easily proved,
that in all forms the outer
antennas in reality issue beneath the inner. The 2 pairs of maxilla?, in the
typical Isopoda, differ somewhat from those in the Amphipoda,
the anterior ones
being generally devoid of palp, whereas the posterior ones carry outside the outer
lobe a lamellar appendage which ought to be regarded as a palp. The maxillipeds
only exhibit a single pair of masticatory lobes, answering to the basal lobes in the
Amphipoda. On the other hand they are provided outside with a more or less
distinctly developed epignath, wholly wanting in the Amphipoda. In parasitic
forms, as usual, the oral parts become more or less modified in their struc-
ture. The appendages of the mesosome, the legs, exhibit only in the terrestrial
Isopoda (Oniscoidea) such a uniform appearance as to justify the name
given to the order; but in by far the greater part of the Isopoda, the structure
of the legs is rather diversified, in some cases (for instance in the Munnopsidee)
even more so than in the Amphipoda. The 1st pair generally differ conspicuously
from the next succeeding pnes, being prehensile and applied to the oral region,
1 Crustacea.