THE ANIMAL KINGDOM 



15 



Class Arachnoidea 



This division of the arthropods, most of them air-breathing, but 

 without tracheae, in addition to a vast number of minute creatures, 

 contains the harvest men, the scorpions, and over ten thousand species 

 of spiders all with four pairs of walking legs. 



The familiar horseshoe or king crab, Limulus, among the largest of 

 arthropods, in length up to 50 cm. is the only surviving relative of the 

 Paleozoic trilobites, which, in their day, dominated the sea. The newly- 

 hatched limulus is strikingly like a trilobite; and yet, the limulus is clearly 

 a much modified spider. 



l.3j^ it^»ll»!Sfe ^\M^«IH^»^ 



LIMULUS - 

 AN ARACHNID 



Fig. 10. — A young Limulus viewed from the dorsal side. William Patten and 

 Gaskell have attempted to demonstrate that this animal lies in the direct line of verte- 

 brate ancestry. 



The Limulus has, moreover, a further interest, in that it also resembles, 

 superficially, certain of the Paleozoic ostracoderms, a group which, it is 

 maintained by Patten and Gaskell, contains the long-sought ancestor of 

 all the vertebrates. Limulus belongs to the order Xiphosura (Mero- 

 stomata). The Eurypterids are a closely related group of extinct paleozoic 

 arachnids of large size. 



Class Tracheata 



In addition to the insects, all of which as adults have three pairs of 

 walking legs, the group contains many difi^erent primitive forms which 

 link the arthropods to the annelids. Of these forms, the most primitive 

 is the genus Peripatus, with fourteen to forty-three pairs of legs, and so 

 strange a body that it has been taken both for a mollusc and for a worm. 

 The familiar centipedes and myriapods have from a dozen to nearly two 



