PERACARIDA 



355 



has a depressed, oval body^ the cephalothorax, formed by fusion of the 

 I St thoracic somite with the true head, lying in a notch on the an- 

 terior edge of the 2nd somite of the thorax. Two large, sessile com- 

 pound eyes take up the sides of the head. The abdomen continues the 

 outline of the thorax, and its 6th somite is fused with the telson. The 

 antennules, which are usually short in isopods, are here minute. The 

 antennae are of a good length, which is due to the elongation of the 

 two joints which precede the flagellum. The mandibles^ unlike those 

 of most isopods, lack the palp, but otherwise they are complicated, 

 having between the incisor and molar processes a row of spines and the 

 movable structure known as the lacinia mobilis (Fig. 261 A, la.mo.) 

 which is characteristic of the Peracarida. The maxillules and maxillae 

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



Fig. 260. Dorsal and ventral views of Ligia oceanica. 

 From the Cambridge Natural History. 



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 and 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 



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