202 



THE LIFE OF CRUSTACEA 



these channels are nowhere open to the outside, the 

 air must find its way in by diffusion through the 

 cuticle. 



Even more abundant than Oniscus asellus, an 

 often found together with it, is Porcellio scaber (see 

 Fig. 20, p. 51). It is usually of a dark bluish-grey, 

 but occasionally it is irregularly mottled with 



FIG. 64 STRUCTURE OF THE BREATHING ORGANS OF Porcellio scaber. 

 (From Lankester's "Treatise on Zoology^" after Stoller.) 



A, Exopodite of first pleopod, showing the tuft of air-tubes 

 ("pseudo-tracheae"), seen through the transparent cuticle; B, 

 vertical section through same ; C, part of section more highly 

 magnified, art, Point of attachment of exopodite to peduncle ; 

 c, cuticle; gr, grooved area of cuticle ; hy, hypodermis, or layer 

 of cells under the cuticle ; n, nucleus of hypodermis cell of air- 

 tube ; o, external opening ; tr, air-tubes 



lighter colour. The flagellum of the antenna has 

 only two segments. The most interesting difference 

 from Oniscus, however, is found in the pleopods. If 

 the under-side of the living animal be examined with 

 a pocket lens, a white spot will be seen on each 



