Fig. 15.40. Class Xipho- 

 surida : Limulus polyphemus, the 

 horseshoe crab of the Atlantic 

 coast of North America. 

 (Redrawn from R. E. Snod- 

 grass, Textbook of Arthropod 

 Anatomy, copyright 1952 by 

 Cornell University Press, 

 printed by permission.) 



THE PHYLA ARTHROPODA AND ONYCHOPHORA 

 ■ Prosoma 



Operculum 



crab they are unmodified walking legs; in scorpions they form the conspicuous 

 great chelae used in capturing prey; in male spiders they are often very com- 

 plex structures adapted for the transfer of spermatozoa to the female at 

 mating. The remaining four pairs of appendages are relatively undifferen- 

 tiated walking legs. Only the horseshoe crab, Limulus, exceptional in many 

 respects, bears segmental appendages on the opisthosoma (Fig. 15.40). 



Most modern chelicerates are small, terrestrial animals. Limulus is an 

 exception to this generalization also; it is a marine form, often reaching 2 

 feet in length. Fossil remains of forms ancestral to Limulus show that this 

 genus has remained almost unchanged since Devonian times, some 350,000,000 

 years ago. In correlation with its marine habitat, Limulus bears on the 

 ventral side of the opisthosoma many leaf-like gills which function in oxygena- 

 tion of the blood. These delicate gills are protected by plate-like expansions 

 of the opisthosomatic appendages, used also for swimming. Among terrestrial 

 arachnids, scorpions and most spiders have developed internal cavities termed 

 book lungs, containing many thin, hollow plates between which the blood 



Atrium 



HI A. // — ^i" 



Sternum 



spiracle 



Lamellae 



Fig. 15.41. Book lung of a scorpion. The hollow lamellae, filled with air, communicate with 

 the cavity of the atrium; blood circulates between the individual lamellae. (Redrawn from 

 R. E. Snodgrass, Textbook of Arthropod Anatomy, copyright 1952 by Cornell University Press, 

 printed by permission.) 



481 



