398 



THE INVERTEBRATA 



The air tubes on the abdominal limbs have been alluded to on 



P-348. 



Asellus (Fig. 276 A), the hog slater, is a common freshwater crus- 

 tacean. It differs from Ligia, among other ways, in having all the 

 abdominal somites fused, a flagellum on the antennule, a palp on the 

 mandible, and free coxopodites on the legs. 



Idotea, common among weeds, etc., on the British coast, differs 

 from Ligia in having the last four abdominal somites fused with the 

 telson and the uropods turned inwards as valves to cover the pleopods. 



cp.- 



Fig. 275. Limbs of Li^g/a. A, Mandible. B, Maxillule. C, Maxilla. D, Max- 

 illiped. E, Third abdominal limb. cp. coxopodite; en. endopodite; ex. exo- 

 podite; mc. incisor process; la.mo. lacinia mobilis; niol. molar process; 

 pr. protopodite; spi. spine row. i & 2 first two joints, fused; i', 3', endites. 



Many of the Isopoda are parasitic. Among these there is found 

 every grade from well-organized temporary parasites to some which 

 are as adults mere sacs of eggs. Aega (Fig. 276 B), a fish louse, has the 

 ordinary isopod form, though heavily built, and with piercing mouth 

 parts and some of the legs hooked. Its broad uropods form a tail fan. 

 Bopyrus (Fig. 277 A), in the gill chamber of prawns, with dwarf males, 

 is more degenerate but still recognizable as an isopod. Cryptoniscus 

 (Fig. 277 B), a " hyperparasite " on members of the Rhizocephala and 

 a protandrous hermaphrodite, is extremely degenerate. Many of these 

 parasites produce parasitic castration (see p. 383). 



