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Tribe k.
ASELLOTA.
Be-mark*. In the present group of Isopoda all the segments of the meta-
some are fused together, forming a more or less broad shield-like plate, the caudal
segment. The pleopoda have wholly lost their swimming power, being exclusively
branchial in character, and they are moreover considerably reduced in number. In by
far the greater number of the forms, the 1st pair are transformed to a single oper-
cular plate, to protect the extremely delicate succeeding pairs, this operculum
being, in the male, peculiarly modified. The uropoda. as in the group Chelifera,
are terminal, never forming part of a caudal fan. nor being valvular in character.
Though, as above stated, the appendages of the metasome are quite unserviceable
for locomotion, there are some forms belonging to this group, which show them-
selves to be very expert swimmers; but the swimming is here effected in a very
different manner, vix., by the aid of the 3 posterior pairs of legs, which,
in such
cases, are found to be peculiarly modified, forming oar-like swimming implements,
by the strokes of which, the animal is propelled backwards. This is more par-
ticularly the case in one of the families, the Munnopsidce;
but some of the Des-
mosomidce are also enabled to move in a similar manner, though never so rapidly as
the first-named. As to the general form of the body, it is greatly variable, sometimes
very broad and depressed, sometimes slender and elongated, approaching to a cylin-
drical shape. The cephalon is always well defined, and the mesosome exhibits the nor-
mal number of segments, which sometimes arrange themselves with tolerable distinct-
ness into 2 sets by a median constriction. The coxal plates are very small or quite
obsolete, never forming a marginal area, as is generally the case in the two pre-
ceding groups. Of the antenna 1
, the superior ones are generally
smaller than the
inferior, which sometimes attain an excessive length. The oral parts are, on the
whole, normally developed, and of the legs, only the 1st pair sometimes assume a