CRUSTACEA OF THE LAND 



203 





exopodite of the first two pairs of pleopods. When 

 the structure of the pleopods is investigated by 

 means of microscopic sections (Fig. 64), it is found 

 that the white spots are tufts of fine branching tubes 

 radiating into the interior of the exopodite from a sHt- 

 Hke opening on the outer edge. These tubes arise by 

 an in-pushing of the integu- 

 ment, and they are hned 

 throughout by a dehcate con- 

 tinuation of the external cuticle. 

 During hfe they are filled with 

 air, and they serve to aerate the 

 blood circulating in the interior 

 of the appendage. 



Another Woodlouse common 

 in England is Armadillidium 

 vulgare (Fig. 65), a little slaty- 

 grey species with a very con- 

 body, which rolls itself 



Fig 65—Arinadilhdhim 

 viilgme. X 2I. (After 

 Sars.) 



vex 



into a ball when touched. Like the last-mentioned 

 species, it has two segments in the flagellum of its 

 short antennae, and it has tufted air-tubes in the 

 exopodites of the first two pairs of pleopods. It is 

 often mistaken for an animal of widely different 

 structure, which it superficially resembles — the Pill 

 Millipede [Gloineris marginata). The latter, however, 

 may easily be recognized by having either seventeen 

 or nineteen pairs of walking legs (instead of seven 

 pairs), set close together in the middle line of the 



