LIMULUS AN ARACHNID. 519 
drical segments of the Scorpion as something over and above 
and not developed in Limulus at all. 
It seems, however, probable from the evidence of extinct 
forms, as well as from the abortive segmentation of the 
embryo, that Limulus is mo¢ derived from an ancestor in 
which the telsonic area was as limited in its production of 
segments as it is in Limulus itself, but on the contrary, that 
the ancestor of Limulus had the full complement of seg- 
ments (and possibly more) which is seen in Scorpio and the 
Eurypterina. In that case the przanal area and spine of 
Limulus would not merely be an area representing the five 
cylindrical segments and sting of Scorpio in poéentzality, 
but would be the actual representative of those segments 
gradually reduced and fused in the course of an historic 
process of change. 
II. Appendages.—At each stage of the comparison between 
Limulus and Scorpio, the proofs of the intimate affinity of 
the two animals become more convincing, since we find that 
the view which it is necessary to adopt in order to make 
one set of structures agree closely in the two animals, is 
precisely the view which it is necessary to adopt, when a 
second set are considered, in order to make agreement 
possible. 
We have just dealt with the tergites and have found an 
exact correspondence of piece for piece, with the exception 
that four preeanal segments are suppressed or five fused in 
Limulus which are discretely present in Scorpio. In order 
to admit such an agreement of piece for piece as to tergites, 
we have to reject the view that the chilaria and the genital 
operculum represent segments belonging to the cephalo 
thoracic tergite, for in that case the cephalothorax of 
Scorpio would be a fusion of six, whilst that of Limulus 
would be a fusion of eight pieces. 
When we come to examine the sternites, we shall find 
that the exclusion of the chilaria from the series of appen- 
dages is exactly what is required in order to identify the 
sternites of Limulus with those of Scorpio, and the removal 
of the genital operculum of Limulus from the cephalo- 
thorax makes its identity with the genital operculum of 
Scorpio even more obvious than it would otherwise be. 
The six pairs of appendages of the cephalothorax of 
Limulus may be compared one by one with the six pairs of 
Scorpio. 
Cephalothoracic appendage, No. I—We have already 
disposed of the obstacle which has been always raised 
hitherto when the chelicere of the Scorpion have been 
