The development of the simple eyes has not yet been studied.
The simple eyes so far described are always placed on the head, and are usually rather numerous.
(2) Compound eyes. Compound eyes are almost always present in the Crustacea, and are usually found in adult Insects. In both groups they are paired, though in the Crustacea a median much simplified compound eye may either take the place of the paired eyes in the Nauplius larva and lower forms, or be present together with them during a period in the development of higher forms.
The typical compound eye is formed ([fig. 283]) of a series of corneal lenses (c) developed from the cuticle; below which are placed bodies known as the crystalline cones, one to each corneal lens; and below the crystalline cones are placed bodies known as the retinulæ (r) constituting the percipient elements of the eye, each of them being formed of an axial rod, the rhabdom, and a number of cells surrounding it.
The crystalline cones are formed from the coalescence of cuticular deposits in several cells, the nuclei of which usually remain as Semper’s nuclei. These cells are probably simple hypodermis cells, but in some forms, e.g. Phronima, there may be a continuous layer of hypodermis cells between them and the cuticle. In various Insect eyes the cells which usually give rise to a crystalline cone may remain distinct, and such eyes have been called by Grenacher aconous eyes, while eyes with incompletely formed crystalline cones are called by him pseudoconous eyes.
The rhabdom of the retinulæ is, like the crystalline cone, developed by the coalescence of a series of parts, which are primitively separate rods placed each in its own cell: this condition of the retinulæ is permanently retained in the eyes of the Tipulidæ.
The development of the compound eye has so far only been satisfactorily studied in some Crustacea by Bobretzky (No. [367]); by whom it has been worked out in Palæmon and Astacus, but more fully in the latter, to which the following account refers:
Fig. 283. Diagrammatic representations of parts of a compound Arthropod eye. (From Gegenbaur.)
A. Section through the eye.
B. Corneal facets.
C. Two segments of the eye.
c. corneal (cuticular) lenses; r. retinulæ with rhabdoms; n. optic nerve; g. ganglionic swelling of optic nerve.