LIMULUS AN ARACHNID. 523 
in Limulus closely resembles No. 111. As in No. It, an 
epicoxite is present. 
The corresponding appendage of Scorpio has been already 
mentioned. It has seven joints and a large sterno-coxal 
process, but no epicoxite, such as occurs in the limb next in 
front of it. 
Cephalothoracic appendage, No. V.—In Limulus this 
resembles Nos. 111 and 1Vv, like them having an epicoxite. 
In Scorpio, No. v, is a seven-jointed ambulatory limb, 
with large coxa fused to the coxa of the next following 
appendage, but devoid of sterno-coxal process. 
Cephalothoracic appendage, No. VI.—In Limulus this is 
the characteristic digging limb, unlike in the special modifica- 
tion of its parts and their remarkable function (for which 
see the citations of Lockwood and of Lloyd in ‘ Owen’s 
Memoir,’ No. 7) any other arthropod appendage. 
In structure it is remarkable for exhibiting the feature of 
secondary movable arthrites diverging from the axis of the 
limb, unusual in Arthropoda other than the Crustacea. Seven 
axial sclerites or segments can be distinguished, the coxa 
being large, as in the other limbs, but devoid of an epicoxite. 
On the other hand, whilst the “ endite ” is thus absent, an 
“‘exite”’ is developed in the form of a flattened elongated 
piece articulated to the external border of the coxa (wood- 
cut fig. 4, vi ex’ right). 
The second and third segments of the axis are devoid of 
apophyses, but the fourth bears a large spine-like articulated 
endite. The fifth joint of the axis carries four flattened 
apophyses (endites and exites), which are articulated and 
capable of active movement. ‘The sixth joint bears one arti- 
culated endite, and, further, the short terminal seventh or 
ultimate segment of the axis, which is relatively much 
longer in newly hatched individuals than in the adult. 
The sixth cephalothoracic appendage in Scorpio is quite 
similar to the three preceding walking legs. Its large coxa 
is fused to that of the fifth appendage of the same side. The 
spinous outgrowths on the sixth and seventh segments of 
this and the other legs are in character somewhat similar to 
the more highly developed apophyses of the digging limb of 
Limulus. 
The seventh pair of appendages or genital operculum.—In 
Limulus lying between the bases of the sixth pair of cepha- 
lothoracic appendages is a pair of sclerites, the chilaria of 
Owen, actually the metathoracic division of the sternum 
(woodcut fig. 5,s¢. right), which belongs to the segment carry- 
ing the sixth pair of appendages. Precisely similar in position 
VoL, XXI,—NEW SER, MM 
