MALACOSTRACA 



397 



the movable structure known as the lacinia mobilis (Fig. 275 A, la. mo.) 

 which is characteristic of the Peracarida. The maxillules and maxillae 

 are less well developed than those of most isopods. The maxillipeds 

 are broad and close the mouth region from behind. The rest of the 

 thoracic limbs are uniramous and leg-like. Their coxopodites are fused 

 with the body, so that the brood pouch plates (oostegites) of the female, 

 which are epipodites of the legs, seem to arise from the sterna. The 

 first five pairs of abdominal limbs are broad, with plate-like, re- 

 spiratory endopodite and exopodite. The endopodite of the second 

 pair of the male is produced into a copulatory style. The uropods have 

 slender, styliform rami. The alimentary canal has an elaborate pro- 

 ventriculus, adapted, not to chew the food, but to press the juices 



Fig. 274. Dorsal and ventral views of Ligia oceanica. 

 From the Cambridge Natural History. 



out of it and to strain off solid particles from them ; and there are three 

 pairs of mid gut coeca. The heart lies in the hinder part of the thorax 

 and in the abdomen, where blood returns from the respiratory limbs 

 to the pericardium. The nervous system has a concentration of ganglia 

 in the abdomen as well as one for the mouth parts. The gonads are 

 paired, and the testes bear three follicles, characteristic of the Isopoda 

 (see Fig. 276 A). The young when set free from the brood pouch re- 

 semble the adult but lack the last pair of legs. Ligia 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. 



