e, whilst in Scorpio they present
several separate small eyes. The microscopic structure of the central
and the lateral eyes has been shown by Lankester and A.G. Bourne (5)
to differ; but the lateral eyes of Scorpio were shown by them to be
similar in structure to the lateral eyes of Limulus, and the central
eyes of Scorpio to be identical in structure with the central eyes of
Limulus (see below).

[Illustration: FIG. 5.--Entosternum of one of the mygalomorphous
spiders; ventral surface. Ph.N., pharyngeal notch. The posterior
median process with its repetition of triangular segments closely
resembles the same process in Limulus.

(From Lankester, _loc. cit._)]

[Illustration: FIG. 6.--Dorsal surface of the same entosternum as that
drawn in fig. 5. Ph.N., pharyngeal notch.

(After Lankester, _loc. cit._)]

Following the prosoma is a region consisting of six segments (figs. 14
and 15), each carrying a pair of plate-like appendages in both Limulus
and Scorpio. This region is called the mesosoma. The tergites of this
region and those of the following region, the metasoma, are fused to
form a second or posterior carapace in Limulus, whilst remaining free
in Scorpio. The first pair of foliaceous appendages in each animal is
the genital operculum; beneath it are found the openings of the
genital ducts. The second pair of mesosomatic appendages in Scorpio
are known as the "pectens." Each consists of an axis, bearing numerous
blunt tooth-like processes arranged in a series. This is represented
in Limulus by the first gill-bearing appendage. The leaves (some 150
in number) of the gill-book (see figure) correspond to the tooth-like
processes of the pectens of Scorpio. The next four pairs of appendages
(completing the mesosomatic series of six) consist, in both Scorpio
and Limulus, of a base carrying each 130 to 150 blood-holding,
leaf-like plates, lying on one another like the leaves of a book.
Their minute structure is closely similar in the two cases; the
leaf-like plates receive blood from the great sternal sinus, and serve
as respiratory organs. The difference between the gill-books of
Limulus and the lung-books of Scorpio depends on the fact that the
latter are adapted to aerial respiration, while the former serve for
aquatic respiration. The appendage carrying the gill-book stands out
on the surface of the body in Limulus, and has other portions