ASELLIDA. 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—Asellide, Munnopside, Arcturide, 
Idoteide, and Spheromide. 
Family—ASELLID A. 
THE animals composing this family are of a sub- 
depressed form; and have not the posterior pair of 
pleopoda developed into an operculum to the remainder 
of the respiratory apparatus (in which respect they differ 
from the Idoteide and Arcturide, whilst the Spheromide 
have the body very convex, and capable of being rolled 
up into a more or less perfect ball). The mandibles 
ave 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 maxille 
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. 
