44 



THE BRITISH WOODLICE. 



Circulatory system — The blood being aerated in the 

 abdominal appendages, we find that the heart is situated towards 



the hinder end of the body (fig. 18). 

 &&77iacfc Three main arteries supply the 



thorax and head, while the 

 blood is brought from the gills 

 to the heart. 



Excretory system. — The ex- 

 cretory organs consist of a (a) 

 pair of so-called " shell glands," 

 which are considered to be the 

 equivalents of the excretory 

 tubes or nephridia of annelid 



HEART 



Jfy&r&h; yfakafr 



FIG. 17. — THE ALIMENTARY CANAL. 



{Oniscus asellus.) 



worms. In the woodlouse 

 these excretory organs open 

 on the second pair of maxillae. 

 They are composed of a 

 tube {sacculus) closed at one 

 end and more or less bent upon 

 itself (5, p. 261) which com- 

 municates with a labyrinth that 

 is provided with an excretory 

 orifice. Matters are eliminated 

 by the epithelial cells [the his- 

 tology has been described and 



figured in Ligidium hypnerum (66)] , which are very large in Ligia 

 oceanic a. 



CLvleny 



FIG. IS.— THE CIRCULATORY SYSTEM. 



(Oniscus asellus.) 



