ASELLIDJE. 313 



Tribe LIBERATICA. 



THE remainder of the aquatic Isopoda are free in 

 their habits, not residing as parasites on the bodies of 

 fishes or other sea animals. Their legs are consequently 

 not formed for prehension or attachment to their prey, 

 nor armed with a strong hook at the extremity, but, on 

 the contrary, they are slender, and generally of uniform 

 size and structure, and fitted for locomotion. They con- 

 stitute five families Asellidcs, Munnopsidce, Arciuridce, 

 Idoteidce, and Sph&romidae. 



Family ASELLIM;. 



THE animals composing this family are of a sub- 

 depressed form ; and have not the posterior pair of 

 pleopoda developed into an opereulum to the remainder 

 of the respiratory apparatus (in which respect they differ 

 from the Idoteida and Arcturida, whilst the Sphceromida 

 have the body very convex, and capable of being rolled 

 up into a more or less perfect ball). The mandibles 

 are armed with a palpiform three-jointed appendage ; the 

 first two or basal joints of the foot-jaws are strongly 

 produced on the inner margin, and the outer maxillae 

 terminated by three large flattened membranous plates. 



The genus Limnoria differs in several important re- 

 spects from the other genera of this family, and was in 

 fact considered by Dr. Leach to form a separate class 

 amongst his Cymothoades. 



