260 



ARTHROPODA 



[CH. 



enclosed in respiratory chambers. In other land forms tracheae 

 assist the respiratory chambers and in still others entirely replace 

 them. The gills have a peculiar form found only amongst 

 Arachnida. They consist of " books " of thin superposed lamellae 

 attached to the posterior aspect of an appendage. When modified 

 for breathing air these "books" are called lung-books. When, 

 as is the case in Limulus, they breathe oxygen dissolved in water 

 they are called gill-books. The genital orifice is usually on the 

 anterior end of the abdomen and ventral : the group is bisexual. 

 Many different orders are included in the Arachnida, the best 

 known being perhaps those which include the Spiders, the Harvest- 

 men, the Mites and the Scorpions. The last named are found only 

 in warm climates and Mites are too small for investigation with the 

 naked eye, so that we shall take the Spider as an example of 

 Arachnid structure. 



Order I. Araneida. 



Spiders belong to the Order Araneida (Lat. aranea, a spider), 

 in which the abdomen is unsegmented and soft. The second pair 



FIG. 113. The Garden Spider, Epeira diademata, sitting in the centre of its web. 



After Blanchard. 



