ARACHNIDA 



455 



structure and function, consisting of six joints, the basal one being 

 produced into a prominent spiny gnathohase ; they are chelate (except 

 the adult males which are clawed). The last (sixth) pair of appendages 

 has four spines springing from the end of the last joint but one ; while 



Fig. 335. Limulus Polyphemus , the king crab, dorsal view, X i. From Shipley 

 and MacBride. i, carapace covering prosoma; 2, opisthosoma; 3, caudal 

 spine; 4, median eye; 5, lateral eye. 



the four anterior legs are used for walking as well as masticating, this 

 pair is particularly concerned with digging. They also possess an 

 external spatulate process which is inserted under the operculum and 

 cleans the gill books. 



