such unit is termed
an "ommatidium." The lateral eyes of Scorpio consist of groups of
separate small lenses each with its ommatidium, but they do not form a
continuous compound eye as in Limulus. The ommatidium (soft structure
beneath the lens-unit of a compound eye) is very simple in both
Scorpio and Limulus. It consists of a single layer of cells,
continuous with those which secrete the general chitinous covering of
the prosoma. The cells of the ommatidium are a good deal larger than
the neighbouring common cells of the epidermis. They secrete the
knob-like lens (fig. 22). But they also receive the nerve fibres of
the optic nerve. They are at the same time both optic nerve-end cells,
that is to say, retina cells, and corneagen cells or secretors of the
chitinous lens-like cornea. In Limulus (fig. 23) each ommatidium has a
peculiar ganglion cell developed in a central position, whilst the
ommatidium of the lateral eyelets of Scorpio shows small intermediate
cells between the larger nerve-end cells. The structure of the lateral
eye of Limulus was first described by Grenacher, and further and more
accurately by Lankester and Bourne (5) and by Watase; that of Scorpio
by Lankester and Bourne, who showed that the statements of von Graber
were erroneous, and that the lateral eyes of Scorpio have a single
cell-layered or "monostichous" ommatidium like that of Limulus. Watase
has shown, in a very convincing way, how by deepening the pit-like set
of cells beneath a simple lens the more complex ommatidia of the
compound eyes of Crustacea and Hexapoda may be derived from such a
condition as that presented in the lateral eyes of Limulus and
Scorpio. (For details the reader is referred to Watase (11) and to
Lankester and Bourne (5).) The structure of the central eyes of
Scorpio and spiders and also of Limulus differs essentially from that
of the lateral eyes in having two layers of cells (hence called
diplostichous) beneath the lens, separated from one another by a
membrane (figs. 24 and 25). The upper layer is the corneagen and
secretes the lens, the lower is the retinal layer. The mass of soft
cell-structures beneath a large lens of a central eye is called an
"ommatoeum." It shows in Scorpio and Limulus a tendency to segregate
into minor groups or "ommatidia." It is found that in embryological
growth the retinal layer of the central eyes forms as a separate
po