I. CRUSTACEA: ISOPODA 



385 



Order II. Isopoda. 



The Isopoda are readily distinguished from the Amphipoda by their 

 depressed (horizontally Hattened) bodies. The feet are adapted for 

 creeping, and a brood pouch is formed as in the Amphipoda; gills are 

 lacking on the thorax. In the abdomen, the somites of which exhibit a 



Fig. 412. — Asellus aquaticus (from Ludwig-Leunis). a\ a', antennae; tc, brood 

 pouch; fe, pleopoda modified to gills; md, mandibles; p^-p'' , thoracic feet; pa^-pa", 

 abdominal feet (pleopoda); I-VI, head; VII-XIII, thoracic segments; XIV-XX, 

 abdominal segments partly fused. 



great tendency to fusion, the sixth somite bears, in the walking forms, long 

 forked appendages (fig. 412); in the swimming species (414, C) they are 

 flattened and, with the telson, make a swimnung organ. The five anterior 

 pairs of pleopoda are modified for respiration (Fig. 412, k), the endop- 





m^m^ 



Irx 



Fig. 413. — Entoniscus porceUana (from Gerstacker, after Miiller). ,4, male; B, 

 female; C, heart; he, liver; la. brood lamellae; n', ovary. 



odites being thin-walled plates, while the exopodites and the whole first 

 pair serve as opercula or gill covers. As a result of this position of the 

 gills the heart (usually with two pairs of ostia) is abdominal in position. 



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