LIMULUS AN ARACHNID. 529 
Furthermore, it is important to notice that in Scorpio 
neither in the embryo nor at any other time does the seventh 
abdominal segment (thirteenth of the whole series) carry a 
pair of appendages, nor do any of the subsequent cylindrical 
segments. Similarly in Limulus no appendages or rudi- 
ments of appendages are to be detected after the last pair 
of lamelligerous organs—the twelfth of the whole series. 
The segmented region, devoid of appendages in the 
Scorpion, is represented by an unsegmented region devoid 
of appendages in the King Crab. 
Before entering into a more minute comparison of the 
lamelligerous appendages of the Scorpion with those of 
Limulus, with the object of establishing the identity of 
origin of the two series by the detection of agreement 
between them in details of structure, it will be most con- 
venient to examine another series of skeletal elements, 
namely, the sternites. 
III. Sternites.—In Limulus, in the cephalo-thoracic region, 
we find that the integument of the sternal area, though to a 
large extent soft and devoid of hard chitinous plates, yet 
presents here and there well-marked sclerites. On the 
sub-frontal area, a small discoidal piece, the sub-frontal 
sclerite is found (Pl. XXVIII, fig. 4, sf). Between the 
mouth and the bases of the first pair of appendages a much 
more important sclerite occurs, to which the term used by 
Latreille for the similarly placed sclerite in Arachnida, viz. 
(camerostome), may be used. 
In the Scorpion (fig. 8, in front of the mouth to which the 
line m points) a similar tubercular sclerite is found. There 
is advantage in not merely designating this piece “ labrum,” 
since there is but little ground for holding it to be equivalent 
either to the labrum of Insecta or to that of Crustacea. 
In the Spider Mygale (fig. 9) and in Galeodes (figs. 10 
and 11, cam), this same piece is observed, attaining a remark- 
able development in the latter. 
When we come to the region behind the mouth, we find 
in Limulus a large median sclerite extending from the 
pharynx backward. It lies between the bases of the third, 
fourth, fifth, and sixth pairs of cephalothoracic appendages. 
On account of its position, it may be termed the thoracic 
promeso-sternite (Pl. XXVIII, fig. 4, pmst), since it appears 
to represent elements which, in other Arachnida, are marked 
off as distinct prosternite and mesosternite. 
In Scorpio we find nothing corresponding to this piece. By 
the enlargement and mesiad production of the coxe of the 
four hinder cephalothoracic appendages it has been as it were 
