482 



CRUSTACEA MALACOSTRACA. 



are branched outgrowths from the margins of the endopodites of the 

 pleopods, increasing the respiratory surf ace. In the parasitic Bopy- 

 rina the pleopods may be reduced to simple plates, or, as in lone, 

 produced into long filiform and, in the female, branched appen- 

 dages. Finally, among 

 the terrestrial Oniscidae, 

 some genera (Porcellio 

 and Armadittium) have 

 acquired the power of 

 breathing air, the exo- 

 podites of two or more 

 of the anterior ab- 

 dominal limbs being 

 penetrated by numbers 

 of minute tubular air 

 spaces (Fig. 297), open- 

 ing at a notch on the outer margin, and 

 causing a shining white patch visible to 

 the naked eye ; the endopodites of some 

 of the limbs meanwhile retain the deli- 

 cate lamellar character, enabling the 

 animal, while in moist places, to breathe 

 oxygen in solution. 



The central nervous system resembles 

 that of the Gammaridae among the Am- 

 phipods. In Oniscus the nerves to the 

 mouth parts arise from the oesophageal 

 commissure. 



Eyes are usually present and consist of 

 few or many elements, grouped together, 

 in varying degrees of closeness, but they 

 differ from those of the Amphipoda in the 

 fact that each element is provided with 

 a lenticular cornea. Thienemann * has 



shown that a pair of otoeysts, each opening by a narrow 

 orifice to the exterior, is present in the telson of Anihura. 

 In the nearly allied Paranihura they are absent. 



The alimentary canal in most cases resembles that of the 

 Amphipods, but rectal diverticula are not present in Isopods. 

 * Statocysten bei Anthura gracilis, Zool. Anzeiger, xxvi. (1903), p. 406. 



FIG. 297. Exopodite of first 

 right abdominal appen- 

 dage of Porcellio scaber. A 

 from the dorsal side. B in 

 section along the line s-s 

 in A (after Stoller). 1 ar- 

 ticulation of exopodite 

 with protopodite ; 2 the 

 "respiratory tree"; 3 

 blood cavity ; 4 opening of 

 the respiratory tree ; 5 its 

 cavity. 



