A, Diagram of anterior extremity of an early embryo of
_Scolopendra_, ventral view; cl, clypeus; lb, labrum; m, mouth; p.a,
preantennal appendage; a, antenna; int, premandibular rudiment; mdl,
mandible; mx, maxilla; p.g, palpognath; t.g, toxicognath; lg. 1,
first pair of walking legs.
B, Posterior end of a later embryo of _Scolopendra_, ventral view,
showing the anal segment or telson (t); the legs of the last pair in
the adult (lg. 21) and the two rudimentary pairs of legs (lg. 22,
lg. 23).]
Eyes are frequently absent. When present they may be either simple or
compound, i.e. consisting externally of a single lens (monomeniscous)
of or an aggregation of lenses (polymeniscous). Simple eyes vary in
number on each side of the head from one, as in _Henicops_, to many as
forty, as in some species of _Lithobius_. In _Scolopendra_, where
there are four, the corneal lens is a biconvex thickening of the
cuticle. The soft or retinal portion of the eye beneath the lens
consists of an aggregation of large cells forming a single layer
continuous with the epidermic cells of the circumocular area. Thus the
eye is monostichous. The arrangement of the cells, however, is
peculiar. They are invaginated to form what may be described as a very
deep cup with exceedingly thick walls and correspondingly narrow
median space, the outer surface of the cup being formed by the inner
or proximal ends of the cells and the inner surface by their outer or
distal ends. It results from this arrangement that the cells forming
all but the bottom of the invagination lie horizontally, i.e. at right
angles to the vertical axis of the eye. From the distal ends of the
cells are secreted chitinous rhabdomeres, forming a rhabdom which
occupies and fills up the central portion of the cup beneath the
middle of the corneal lens. The outer ends of the cells are nucleated
and are continuous with the fibres of the optic nerve, which passes
from the outer surface of the bottom of the cup to the brain. Compound
eyes are found only in the _Scutigeridae_. Externally the eye consists
of one hundred or more little lenses or lenticles. The retinal portion
is composed of a corresponding number of ocular units or ommatidia.
Each ommatidium is an elongated cone with its broad extremity abutting
against the corneal lenticle. It consists of a non-nucleated
crystalline cone developed from embryonic cells, and is enveloped in
three tiers of large nucleated cells. The cells of the outermost tier
are heavily pigmented; those of the middle and innermost (proximal)
tiers, the retinal cells, are at their inner extremities produced into
threads continuous with the fibres of the optic nerve. In the space
between these cells and the crystalline cone which they surround,
there is a layer of rhabdomeres deposited apparently by the cells.
[Illustration: A and B after Heymons, _Bibl Zool_, 1901, by permission
of E. Nagele.