356 



THE INVERTEBRATA 



mouth parts. The gonads are paired, and the testes bear three folHcles, 

 characteristic of the Isopoda (see Fig. 262). The young when set free 

 from the brood pouch resemble the adult but lack the last pair of 

 legs. Ltgia is omnivorous, but chiefly eats Fucus. It gnaws with 

 its mandibles, feeding hurriedly at low tide. 



Armadillidium, the common woodlouse, is more completely ter- 

 restrial in its habits than Ligia. Its antennae and uropods are short 

 and thus permit the body to roll up into a ball in the familiar manner. 

 The air tubes on the abdominal limbs have been alluded to on p. 311. 



Fig. 261. l^xmhsoi Ligia. A, Mandible. B, Maxillule. C, Maxilla. D, Maxil- 

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

 podite; inc. incisor process; la.?no. lacinia mobilis; tnol. molar process; 

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



Asellus (Fig. 262 A), the hog slater, is a common freshwater 

 crustacean. 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 covering valves for the 

 pleopods. 



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. 262 B), a fish louse, has the 



