THE ISOPODA 



203 



ment. This condition is found in the Cymothoidae, Serolidae, 

 some Sphaeromidae, and in the Tylidae among the Oniscoidea. 

 When the suture line disappears, as in most Oniscoidea (Fig. 

 118), it is impossible to distinguish the coxal plate from a true 

 pleuron. In all Isopoda, however, with the single exception 

 of the genus Plakarthnum (Sphaeromidae), the coxopodites of 

 the second thoracic somite (the first free somite) are completely 

 coalesced vith the body. 



The ischiop lite of the thoracic legs is generally more or less 

 elongated, not vary short as it 

 is in Tanaidacea and Amphi- 

 poda. The dactylopodite gener- 

 ally ends in a stout claw which 

 may bo completely coalesced 

 with :he S3gment or defined 

 from it by i moure. In Janira 

 and some o;her Asellota, how- 

 ever, there are several stout 

 claw-like spines. 



Very commonly one or 

 more of the anterior pairs of 

 thoracic legs assume a pre- 

 hensile function and become 

 more or less completely sub- 

 chelate, through never forming 

 a perfect chela such as is 

 found among the Tanaidacea. 

 The most perfect natatory legs 

 are found in Munnopsis and 

 related genera, where the last 

 three pairs have the carpus and FIG. 1-24. 



propodus expanded into OVal fyige brandiialls (Bopyridae). o, female, seen 



iji F . -, .,, i from ventral side; Jlrl, oostegite ; A', abdominal 



paddles iringed With long Setae, appendages. 6, abdomen of same with adhering 



Tn ///>cmv>}/<? /ViilvifAri\ male ( <J ), further enlarged. (After Cornalia and 



in AmeSOpOUB (NdlVllera; rancerj> from Class's Textbook.) 



the fourth and fifth pairs of 



thoracic appendages (first and second peraeopods) are unrepresented, 

 except by the oostegites in the female, a condition curiously repeat- 

 ing that found in the Caprellidea among Amphipoda. 



The oostegites (Fig. 124, 2>rl), which, in the great majority of 

 Isopoda as in other Peracarida, form the brood-chamber of the 

 female, are clearly seen in the Asellota to be attached to the 

 coxopodites of certain of the thoracic legs. In the other tribes 

 the coxopodites are more or less completely coalesced with the 

 corresponding somites and the oostegites appear to spring from 

 the ventral surface of the body close to the bases of the legs. In 

 certain Cymothoidae and Epicaridea a pair of oostegites is present 



