uch, which is pushed in laterally beneath the corneagen layer from
the epidermic cell layer. Hence it is in origin double, and consists
of a true retinal layer and a post-retinal layer (fig. 24, B), though
these are not separated by a membrane. Accordingly the diplostichous
ommatoeum or soft tissue of the Arachnid's central eye should strictly
be called "triplostichous," since the deep layer is itself doubled or
folded. The retinal cells of both the lateral and central eyes of
Limulus and Scorpio produce cuticular structures on their sides; each
such piece is a rhabdomere and a number (five or ten) uniting form a
rhabdom (fig. 26). In the specialized ommatidia of the compound eyes
of Crustacea and Hexapods the rhabdom is an important structure.[2] It
is a very significant fact that the lateral and central eyes of
Limulus and Scorpio not only agree each with each in regard to their
monostichous and diplostichous structure, but also in the formation in
both classes of eyes of rhabdomeres and rhabdoms in which the
component pieces are five or a multiple of five (fig. 26). Whilst each
unit of the lateral eye of Limulus has a rhabdom of ten[3] pieces
forming a star-like chitinous centre in section, each lateral eye of
Scorpio has several rhabdoms of five or less rhabdomeres, indicating
that the Limulus lateral eye-unit is more specialized than the
detached lateral eyelet of Scorpio, so as to present a coincidence of
one lens with one rhabdom. Numerous rhabdomeres (grouped as rhabdoms
in Limulus) are found in the retinal layer of the central eyes also.

[Illustration: FIG. 14.--The first three pairs of mesosomatic
appendages of Scorpio and Limulus compared.

VII, The genital operculum.
VIII, The pectens of Scorpio and the first branchial plate of
Limulus.
IX, The first pair of lung-books of Scorpio and the second branchial
plate of Limulus.
gp, Genital pore.
epst, Epistigmatic sclerite.
stg, Stigma or orifice of the hollow tendons of the branchial plates
of Limulus.

(After Lankester, _loc. cit._)]

Whilst Limulus agrees thus closely with Scorpio in regard to the eyes,
it is to be noted that no Crustacean has structures corresponding to
the peculiar diplostichous central eyes, though these occur again
(with differences in detail) in _Hexapoda_. Possibly, however, an
investigation of the development of the median eyes of