128 



CRUSTACEA PEKACARIDA 



(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, Mannopds) 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 tlie abdomen 

 is fully segmented, the pleopods are respiratory, their endopodites 

 being delicate branchiae, while their exopodites are plate-like and 



Flu. 84. — Llgia ocmnica, ventral and dorsal views, x 1. (From original drawings 

 prepared for Professor Weldou. ) 



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, e.g., 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 AVebb and Sillem (1906) may be 

 lirofitably consulted. Budde Lund's Isopoda Terrostria, 1900, is useful to the 

 specialist. 



~ The pleopods are traversed by a system of minute tubes called pseudotracheae, 

 somewhat resembling the tracheae of Insects. 



