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.’ 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 Z. polyphemus 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 Z. 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 LZ. 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 hes 
prone on the bottom of the sea. To quote Mr. Lloyd,’ 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. Mier. Sci. xxi., 1881, p. 504. 
2 Tr. Linn. Soc. xxviii., 1872,"p. 471. 
