516 PROFESSOR E, RAY LANKESTER, 
Abdominal tergites—Following upon the cephalothoracic 
plate we have in the Scorpion seven wide band-like sternites, 
to which succeed five narrow cylinders, the dorsal part of 
each of which is tergite, and solidly fused with the ventral 
half or sternite. In the last of these twelve segments is 
placed the anus (in fig. 2 its position is marked, though it 
is not seen on account of its ventral position), and beyond 
the anus is the postanal spine or sting. 
In Limulus (fig. 1), in place of the seven band-like and 
five half-cylindrical tergites, we find one large chitinous 
plate, which is known as the “ abdominal carapace.” In its 
posterior region is placed the anus, and to it succeeds a post- 
anal spine, sometimes, but erroneously, compared to the cylin- 
drical segments of the Scorpion’s body. Clearly enough the 
postanal spines in the two cases correspond to one another. 
If there is correspondence between Limulus and Scorpio of 
segment for segment and piece for piece throughout (as it 
is the purpose of this essay to demonstrate), then in the 
abdominal carapace of Limulus we must find the repre- 
sentatives of the twelve segments, which in the Scorpion 
exist between cephalothorax and anus. The embryonic 
Limulus, as has been shown by Dohrn and Packard, ex- 
hibits in this region of the body a series of separated seg- 
ments, which fuse together as growth advances, and constitute 
the one immovable abdominal carapace. In the adult the 
indications of the former existence of these separate seg- 
ments is more Obvious than has been supposed. In fig. 1 
I have indicated by dotted lines the series of ridges, which 
can be made out in the abdominal carapace of an adult 
Limulus polyphemus, and which clearly mark off a number 
of the original segments. 
With regard to the general form of this region as com- 
pared with the body of the Scorpion, it may be pointed out 
that here, just as in the region of the cephalothorax, there is 
an excessive development and exaggeration of the margin 
of the dorsal integument, so that the central area marked 
out in the figure is the real “ body” of the Limulus, and 
the wide spreading lateral areas are only enormous excres- 
scences of a relatively superficial character. It is not diffi- 
,cult to find numerous parallels to these pleural develop- 
ments in all groups of Arthropoda. 
Returning to the examination of the actual number of 
segments indicated in the abdominal carapace of the adult 
Limulus, we find areas corresponding to the seven wide 
tergites of the Scorpion marked in the drawing of Limulus 
by the numbers vil to x111. Corresponding to these areas 
