50 



Fam. 2. Gnathiidae. 



Characters. — 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. Mesosome apparently composed of only 5 segments, though a rudi- 

 mentary anterior and posterior segment may be discerned. Metasome narrowing 

 abruptly to a width much narrower than the mesosome, composed of 6 segments, the 

 last triangularly produced behind, 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 number, 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 Apseudidce 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, Pranizidw. 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, Gnathia 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 Kev. Mr. Stebbing has re- 

 cently proposed to raise a most anomalous form described by Mr. Beddard from 

 the Challenger Expedition, to the rank of a distinct genus Euneogncdhia, and he 



