788 



CLASS V. ARACHNID A. 



segments, and are inserted by narrow bases at the antero- 

 lateral angles of the camerostome. The five succeeding pairs 



subserve the 

 double func- 

 tion of masti- 

 cation and 

 loc omotion. 

 Their broad 

 bases of inser- 

 tion are pro- 

 duced intern- 

 ally into spi nose 

 masticatory 

 lobes, which 

 form the lateral 

 boundaries of 

 the extra-oral 

 space. In the 

 female the four 

 anterior loco- 

 motory limbs 

 (II-V) are 

 chelate, while 

 in the adult 

 male (the 

 young males 

 resemble the 

 females) the 



FIQ. 511. Ventral view of Limulus polyphemusx^. 1 cephalo fi r ~f. /TT\ /] n T 



thorax; 2 abdomen; 3 caudal spine; 4 chelicerae ; 5-9 the x 



five legs; 10 genital operculum (1st abdominal appendage) ; 11-15 moluCCCinUS and 

 branchiate appendages (abdominal appendages 2-5) ; 16 anus ; 



17 points to the region of the mouth ; 18 chilaria (7th cephalo- T InnaifMina 

 thoracic appendages) (after Shipley and MacBride). 



the first two 



II and III) end in curved claws. The fifth pair (VI) differs from 

 the others by the possession of a peculiar curved spathulate pro- 

 cess articulated to the outer aspect of its base, by its non-chelate 

 termination, and by the presence of a group of four broad im- 

 bricated spines, which are separately movable, at the distal end 

 of the antepenultimate segment. These are used in shovelling 

 back the mud or sand in which the animals live, and in propell- 

 ing them forward. The spathulate process recalls a crustacean 



