isoroDA. 



457 



a 



they bear two flagella. As in the Amphipoda, pale, plumous setae 

 and olfactory cones are present on the antenna?. The mouth parts 

 are in some parasitic IsojMda modified for piercing and sucking. The 

 mandibles (except in Bopyridce and Oniscidce) often bear a three- 

 jointed palp. On the other hand, the two pairs of maxilla?, which 

 are usually bi- or tri-lobed, are in general without the palpiforrn 

 appendage. The maxillipeds form a sort of underlip, but present 

 great differences in the arrangement of their parts (fig. 358). 



As a rule the seven pairs of thoracic legs are adapted for walking 

 or attachment, and in the female some of them are provided with 

 delicate membranous plates 

 (oostegites) which form a brood 

 pouch. They never bear gills. 

 The branchial function is dis- 

 charged by the delicate inter- 

 nal rami or endopodites of 

 the abdominal limbs (pleo- 

 pods), the anterior pair of 

 which is frequently modified 

 to form a large operculum 

 overlying the following pairs. 

 In certain of the terrestrial 

 Isopocls (Porcellio and Arma- 

 dillo) the opercular plates 

 of the two anterior pairs of 

 abdominal limbs contain a 

 system of air spaces which ap- 

 pear to assist respiration. The 

 heart, unlike that in Amphi- 

 pods, lies (except in Tandidce) 

 in the posterior thoracic seg- 

 ments or in the abdomen. 



The sexes are (except in Cymothoidce) separate, and the position 

 and arrangement of the generative organs correspond in general 

 with those of the Amphipoda. The sexes are distinguished by 

 external sexual characters, which in some cases (Bopyridce) may lead 

 to a strongly-marked dimorphism (fig. 359, a, b). In the male 

 three tubular testes unite on either side to form a dilated seminal 

 vesicle, from which the vasa deferentia are given off. The latter are 

 frequently separate along their whole length and, at the end of the 

 last thoracic segment, each of them enters a cylindrical appendage 



FIG. 359. Gyge IranchMis (after Cornalia ami 

 Panceri). a, Female seen from the ventral 

 side ; Sri, oostegite ; J5T, branchiae. I, 

 Abdomen of the same strongly magnified, 

 with adhering male. 



