LIMULUS AN ARACHNID. 521 
assimilated to the chelicere or first pair of limbs of Limulus. 
Instead of there being a difference as to innervation we 
have seen that there is a real identity. 
In Limulus, each of the first pair of appendages is a 
short pair of nippers (woodcut, fig. 4, 1, right) composed of 
three sclerites; at the base of the two appendages and 
between them and the mouth is placed an ovate sternite, the 
camerostome or upper lip. (Plate XXVIII, fig. 4). 
In Scorpio (woodcut, fig. 4, 1 left) a similarly small pair 
of appendages is found similarly composed of three sclerites, 
and similarly overhanging an oval “ camerostome.” 
Ceph. thor. app., No. II.—In this and the following leg- 
like appendages of Limulus six chief sclerites are developed, 
the basal one or “coxa” being much enlarged, and its in- 
terior border produced into a well-marked process provided 
with tooth-like hairs. The arrangement of the limbs around 
the mouth and the central sternite which follows it (pmsé¢ 
in Pl. XXVIII, fig. 4), is such that the processes of the 
coxze of all ten limbs act together as manducatory organs. 
The process of the coxa may be called “the sterno-coxal 
process ”’ (ste. in the woodcut, fig. 4). The second cephalo- 
thoracic appendage in the female Limulus polyphemus is like 
the third, fourth, and fifth, a chela—that is to say, the penulti- 
mate sclerite is produced so as to form with the last sclerite 
a pair of nippers. In the male this is not the case, the 
second pair of appendages being thicker and heavier than 
in the female, and the penultimate joint not prolonged. The 
form of appendage seen in the male L. polyphemus in this 
position is similar to appendages seen in other Arachnida 
than Scorpio, viz. Thelyphonus (woodcut, fig. 12). 
The second pair of appendages in the Scorpion is like 
that of the female Limulus, but relatively larger. It con- 
sists of six sclerites as in Limulus, and has a sterno-coxal 
process on its coxa, which acts with its fellow of the opposite 
side as a Jaw (woodcut, fig 4, 11). 
Cephalo-thoracic appendage, No. III.—1n Limulus poly- 
phemus this has, in both sexes, the same form as has the 
second appendage in the female. It is similarly composed 
of six sclerites, but in addition to these we find a distinct 
movable sclerite developed on the median border of the coxa. 
This sclerite may be termed the “ epicoxite”’ (woodcut, 
fig. 4, 111, epe., right). The epicoxite is a remarkable 
feature, and is not easily paralleled among Arthropoda. 
The basal “ endite ” of the limbs of the Crustacean Apus is 
similar to it, and perhaps derived from a common ancestral 
origin. 
