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Fam. 2. Gnathiidae.
r*. Body of male and female of very different appearance, being.
in the former, depressed and dilated in front, in the latter, more or less fusiform ;
body of immature animal (larva) also rather different, though more resembling
that of female. Cephalon of male very large, as compared
with that of female
and larva. Mesosomc apparently composed of only 5 segments, though
mentary anterior and posterior segment may
a rudi-
he discerned. Metasome narrowing
ahruptly to a width much narrower than the mesosome, composed of 6 segments, the
last triangularly produced hehind, and forming together with the uropoda
a nor-
mal caudal fan. Antennae comparatively short, and of same structure in the
two sexes, issuing close together from the outer corners of the cephalon.
Oral parts in the adult animal much reduced, consisting in the male of
a pair of mandibles freely projecting in front and a pair of peculiarly modi-
fied maxillipeds, in female of only the latter appendages. Oral parts in the
larva present in the normal numher, and densely crowded together, to form an
apparatus for penetration and suction. Gnathopoda in male valve-like, arching
over the lower face of the cephalon, in female much smaller and more distinctly
segmented, in the larva pediform, terminating in a strong hook. Only 5 pairs
of pereiopoda present, the last pair being absent. Pleopoda of equal structure,
natatory or exclusively respiratory (in the adult animal). No true incubatory
pouch present, the ova undergoing their development within the dilated body-cavity.
Remarks. This is one of the most anomalous families of the order, and
in some points even differs more pronouncedly from the typical Isopoda, than
do the Alwinl'ula' and Tanaidce. The sexual differences are highly remarkable, and
the not yet mature animals differ also so widely from the adults, that they have
long been referred to quite a separate family, Prmuzulfe. The latter lead a true
parasitic existence, as shown by the peculiarly modified oral parts,
whereas the
adults are scarcely at all parasitic. The complete absence of the last pair of pe-
reiopoda affords another very anomalous character, not found in other Isopoda,
except in the recently hatched young.
The family comprises as yet but a single genus, Gnatltia Leach; but it
is most probable, that it will be found appropriate to subdivide this genus into
several distinct genera, as there are some apparently essential differences to be
found on comparing the several species. Indeed, the Rev. Mr. Stebbing has re-
cently proposed to raise a most anomalous form described by
Mr. Beddard from
Hie Challenger Expedition, to the rank of a distinct genus Euneoyttathitt. and he