264 ARACHNIDA XIPHOSURA CHAP. 



same relationship to the mouth of Limulus that the labrum has 

 in Insects and some Crustacea. Posteriorly the mouth is 

 bounded by the " promesosternite," a large median plate which 

 lies between the bases of the ambulatory limbs. The pedipalps 

 and all the ambulatory limbs have their bases directed towards 

 the mouth, their gnathobases or sterno-coxal processes are 

 cushion-like structures covered with spines all pointing inwards 

 and with crushing teeth. They form a very efficient man- 

 ducatory apparatus. The boundary of the mouth is finally 

 completed by the chilaria. 



Certain of the appendages which persist will be described 

 with the functions they subserve, the eyes with the sense-organs, 

 the genital operculum with the generative organs, the gill-books 

 with the respiratory system, but the chelicerae, pedipalpi, and 

 walking limbs, which have retained the functions of prehension 

 and locomotion usual to limbs, merit a little attention. 1 The 

 chelicerae are short and composed of but three joints. They 

 are, like the succeeding segments, chelate, and the chelae of all are 

 fine and delicate like a pair of forceps rather than like a Lobster's 

 claw. In the female L. polypliemus the pedipalp is remarkably 

 like the three ambulatory legs which succeed it, and all four are 

 chelate, but in the adult male the penultimate joint of the pedi- 

 palp is not prolonged to form one limb of the chela, which is 

 therefore absent, and the appendage is thicker and heavier than 

 in the other sex. In L. longispina and L. moluccanus the first 

 walking leg, as well as the pedipalp, ends in a claw and not in 

 a chela ; the immature males resemble the females. The first 

 three walking legs in both sexes of L. polyphemus resemble the 

 pedipalpi of the female, and like them have six joints. The fourth 

 and last pair of ambulatory appendages is not chelate, but its 

 distal joints carry a number of somewhat flattened structures, which 

 are capable of being alternately divaricated and approximated or 

 bunched together. This enables them to act as organs for clearing 

 away sand or mud from beneath the carapace as the creature lies 

 prone on the bottom of the sea. To quote Mr. Lloyd, 2 the " two 

 limbs are, sometimes alternately and sometimes- simultaneously, 

 thrust backward below the carapace, quite beyond the hinder edge 



1 They are described in great detail in Lankester's article, " Limulus an 

 Arachnid," Quart. J. Micr. Sci. xxi., 1881, p. 504. 

 - Tr. Linn. Soc. xxviii., 1872, p. 471. 



