ISOPODA. 



481 



vesicles are present on the antennae, but both anterior and 

 posterior are uniramous. The mouth parts are generally 

 masticatory, but in the parasitic forms they are suctorial, and 

 depart more or less from the typical condition here described. 

 The mandibles bear a three-jointed palp, only wanting in Onisci- 

 dae, Bopyridae, and some other cases. The first maxilla is, on 

 the other hand, without a palp and consists of two lobes, con- 

 nected by a basal " cardo," and the second is plate-like and 

 produced into four or fewer lobes. The closely united maxillipeds 

 borne on a common basal piece (Fig. 296) form a structure like 

 the labium of insects limiting the space about the mouth pos- 

 teriorly. Each half consists of a basal segment (stipes) bearing 

 externally the 

 5- or fewer 

 jointed palp, 

 and produced 

 into a lamina, 

 which may be 

 connected 

 with its fellow 

 by interlock- 

 ing hooks. A 

 large epipo- 

 dial (?) plate 

 (v) is c o n - 

 nee ted with 

 the outer side 

 of the stipes. 



The seven succeeding pairs of thoracic legs are, as a rule, adapted 

 for walking or attachment, w4iile the abdominal appendages are 

 here modified in varying number, to serve as respiratory struc- 

 tures. When so modified, they consist of a basal segment support- 

 ing two rounded lobes, of which the inner has a delicate wall 

 permitting the oxidization of the blood, while the outer, which 

 protects it, is of firmer consistency. An anterior pair often 

 forms an operculvm (e.g. Asellus), while the 6th pair are frequently 

 stylif orm or natatory ; but in the Idotheidae the latter are valvular, 

 folding like doors over the other abdominal appendages. In the 

 deep-sea form Anuropus they are respiratory like the preceding 

 appendages. In another deep-seaf orm , Bathynomus giganteus, there 



FIG. 296. Maxilliped of Asellus aquaticus (after Gerstaecker). la 

 lamina ; m sternite of first thoracic segment ; p palp ; st " stipes " ; 

 v epipodial (?) plate. 



Z III 



I I 



