128 CRUSTACEA—PERACARIDA CHAP. 
(Asellidae) being extremely abundant all over Europe in weed- 
grown ditches, the mud of slowly moving streams, and even on 
the shores of large lakes. They are mostly sluggish in habit, 
but the marine Munnopsidae (Fig. 83, Munnopsis) are expert 
swimmers, the swimming organs being fashioned by the expansion 
and elongation of the thoracic legs. 
Sub-Order 5. Oniscoida. 
The Oniscoida! are terrestrial forms in which the abdomen 
is fully segmented, the pleopods are respiratory, their endopodites 
being delicate branchiae, while their exopodites are plate-like and 
Fic. 84.—Ligia oceanica, ventral and dorsal views, x 1. (From original drawings 
prepared for Professor Weldon. ) 
form protective opercula for the gills, and the uropods are 
biramous and not expanded. The epimera of the segments 
are greatly produced. The terrestrial Isopods, although air- 
breathers,” are dependent on moisture, and are only found in 
damp situations. It seems probable that they have been 
derived from marine Isopods, since the more generalised of 
them, eg., Ligia (Fig. 84), common on the English coasts, are 
only found in damp caves and crannies in the rocks. 
' A useful little book on British Woodlice by Webb and Sillem (1906) may be 
profitably consulted. Budde Lund’s Jsopoda Terrestria, 1900, is useful to the 
specialist. 
° The pleopods are traversed by a system of minute tubes called pseudotracheae, 
somewhat resembling the tracheae of Insects. 
