220 ZOOLOGY FOR MEDICAL STUDENTS chap. 



thin chitinous wall which allows ready respiratory exchange of gas 

 between the blood within and the sea-water without. 



The Scorpions are Arachnids which have forsaken the sea and taken 

 to a terrestrial existence, and their appendages, while agreeing exactly 



met. 



Fig. 99. 



A Scorpion. A, dorsal view. c,e, Central eyes ; l.e, lateral eyes ; mes, mesosoma ; mety meta- 

 soma ; p, prosoma ; t, telson. I. Chelicera ; II, pedipalp ; III-VI, walking legs. 



as a series with those of Limulus, show interesting modifications in 

 detail (Fig. 99). The first appendages (I), as in Limulus, are small 

 chelicerae, and the next five (II-VI) are walking legs. These are more 

 slender than those of Limulus and, excepting II (pedipalp), which is stout 

 and chelate, possess a sharply cla-yved foot. The pedipalps are of special 



