Phylum Arthropoda 511 



A group closely related to the horseshoe crabs are the now extinct 

 eurypterids which swarmed in the seas during the Paleozoic era. These 

 bear a rather strong superficial resemblance to present-day scorpions. 



Fig. 170. — Tlie king crab, Limulus polyphemus. A, Ventral view; 

 B, dorsal view; C, trilobite larva. 



THE CLASS ARACHNIDA 



The members of this group are familiar inasmuch as the spiders, 

 scorpions, daddy longlegs, mites, ticks, and many other forms are all 

 included within it. All are characterized by the fact that the body is 

 divided into the cephalothorax and abdomen, there are six pairs of ap- 

 pendages in the adult, paired chelicerae, paired palpi, and four pairs of 

 walking legs. 



The Anatomy of a Spider. — The most conspicuous of the arach- 

 nids are the omnipresent spiders which all are members of the order 

 Araneae. Their anatomy is fairly typical of that of many other arach- 

 nids (Fig. 169,5). 



The cephalothorax and abdomen are joined by the narrow peduncle. 

 In many species, the abdomen is considerably larger than is the cephalo- 

 thorax. A varying number of simple eyes are present on the anterior 

 portion of the cephalothorax, and ventrally the cephalothorax bears the 



