ARTHROPODA 



'75 



with the abdomen consisting of distinctly separate segments 

 (Fig. 167). This is known as the trilobite stage b< oi 



the resemblance to some of the class of extinct Crusl the 



Trilobitae (Fig. [68) whose remains occur in the Palaeozoic 



rocks. In these same formations are found the fossils of another 



class of Crustacea, the Gigantostraca (Fig. [69), which attained 



a length of over a 



meter, and show many 



points of resemblance 



to the adult Xipho- 



sura, but differ chiefly 



in having a distinctly 



segmented abdomen. 



SUBTYPE II. 



ARACHNID A 



The Arachnida(Gr. 



apd-^vij, spider) are 

 readily distinguished 

 from the preceding 

 subtype by the organs 

 of respiration, which 

 are always adapted to 

 breathing air directly, 

 and from the succeed- 

 ing subtype by the 

 number of append- 

 ages present. This 

 group is represented 

 by the scorpions, 

 spiders, mites, and 

 similar animals, and is 

 therefore essentially terrestrial. The body generally exhibits 

 two regions, a cephalothorax and an abdomen, the former 

 usually much the shorter. There are six pairs of appendages 

 and all are attached to the cephalothorax; of these the first 

 two pairs are modified for seizing food or carrying it t<> the 

 mouth and therefore may be considered mouth parts; the 

 remaining four pairs are walking legs. All the appendages 



FlG. 169. Eurypterus fischeri, from the Silurian. 

 Parker and Haswell's Text-book.) 



(From 



