LIMULUS AN ARACHNID, 533 
They have no representative in Mygale (fig. 9), but here 
Scorpio returns to its allegiance and exhibits a well-developed 
sclerite exactly corresponding to them. The pentagonal 
sclerite wedged between the coxe of the last pair of cephalo- 
thoracic limbs in Scorpio (woodcut, fig. 8, met) clearly 
enough agrees in position precisely with the chilaria of 
Limulus (see also woodcut, fig. 5). It is true that the form 
of the pentagonal metasternite of Scorpio differs from that of 
the two little tubercles of Limulus, but the exclusion from 
the functions of the mouth of the former sufficiently accounts 
for the difference. 
In Thelyphonus (woodcut, fig. 12, s¢.”) a triangular meta- 
sternite corresponding in position to that of Scorpio is found. 
It is exceedingly astonishing that so careful an observer 
as M. Alphonse Milne-Edwards should have suggested, as 
he has done, that the “chilaria” of Limulus correspond to 
the “‘ pectines ”’ of the Scorpion, since the former are in front 
of and the latter are behind the genital operculum. When the 
possibility of such homologies is entertained, it is but a natural 
consequence that the complete series of agreements of segment 
for segment and appendage for appendage which obtains be- 
tween Limulus and Scorpio, should be entirely overlooked. 
When we pass to the abdominal segments we find a very 
considerable difference between Limulus and Scorpio in the 
development of sternites. 
The sternal integument of the region at the base of the 
genital operculum and the gill-bearing appendages, is almost 
entirely soft and free from sclerites in Limulus. In Scorpio, 
on the other hand, whilst the sternal region around the 
genital operculum is soft, a well-developed sternite (woodcut , 
fig. 8 z) is found supporting the pectiniform appendages ; 
and for each of the five following segments a broad band- 
like sternal sclerite is developed. The four anterior of these 
are perforated, each by a pair of slit-like apertures leading 
into four pairs of recesses, in each of which a lamelligerous 
appendage is sunk. The fifth is imperforate, and bears no 
appendage. The segments of the so-called ‘ tail” which 
follow present a complete chitinisation of the integument, so 
that the sternites of each segment is confluent with the tergite. 
When we examine the sternal area of the segments of 
Limulus which carry lamelligerous appendages, we find that 
although the integument is mostly soft and flexible, yet 
there are small sclerites present, and, in fact, stigmata or 
apertures leading into pits corresponding to the stigmata of 
the pulmonary sacs of Scorpio. 
These parabranchial stigmata of Limulus have hitherto 
