The eye of Oceania is simpler than that of Lizzia in the absence of a lens. Claus has shewn that in Charybdea amongst the Acraspeda a more highly differentiated eye is present, with a lens formed of cells like the vertebrate eye.
Mollusca. In a large number of the odontophorous Mollusca eyes, innervated by the supraœsophageal ganglia, are present on the dorsal side of the head. These eyes exhibit very various degrees of complexity, but are shewn both by their structure and development to be modifications of a common prototype.
The simplest type of eye is that found in the Nautilus, and although the possibility of this eye being degenerated must be borne in mind, it is at the same time very interesting to note (Hensen) that it retains permanently the early embryonic structure of the eyes of the other groups.
It has ([fig. 277] A) the form of a vesicle, with a small opening in the outer wall, placing the cavity of the vesicle in free communication with the exterior. The cells lining the posterior face of the vesicle form a retina (R); and are continuous with the fibres of the optic nerve (N.op). We have no knowledge of the development of this eye.
Fig. 277. Three diagrammatic sections of the eyes of Mollusca. (After Grenacher.)
A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod.
Pal. eyelid; Co. cornea; Co.ep. epithelium of ciliary body; Ir. iris; Int, Int1 ... Int4. different parts of the integument; l. lens; l1. outer segment of lens; R. retina; N.op. optic nerve; G.op. optic ganglion; x. inner layer of retina; N.S. nervous stratum of retina.
In the Gasteropods the eye ([fig. 277] B) has the form of a closed vesicle: the cells lining the inner side form the retina, while the outer wall of the vesicle constitutes the cornea. A cuticular lens is placed in the cavity, on the side adjoining the cornea. This eye originates from the ectoderm, within the velar area, and close to the supraœsophageal ganglia, usually at the base of the tentacles. According to Rabl (Vol. II. No. 268) it is formed as an invagination, the opening of which soon closes; while according to Bobretzky (Vol. II. No. 242) and Fol it arises as a thickening of the epiblast, which becoming detached takes the form of a vesicle. It is quite possible that both types of development may occur, the second being no doubt abbreviated. The vesicle, however formed, soon acquires a covering of pigment, except for a small area of its outer wall, where the lens becomes formed as a small body projecting into the lumen of the vesicle. The lens seems to commence as a cuticular deposit, and to grow by the addition of concentric layers. The inner wall of the vesicle gives rise to the retina.
The most highly differentiated molluscan eye is that of the Dibranchiate Cephalopoda, which is in fact more highly organized than any other invertebrate eye.
A brief description of its adult structure[183] will perhaps render more clear my account of the development. The most important features of the eye are shewn in [fig. 277] C. The outermost layer of the optic bulb forms a kind of capsule, which may be called the sclerotic. Posteriorly the sclerotic abuts on the cartilaginous orbit, which encloses the optic ganglion (G. op); and in front it becomes transparent and forms the cornea Co, which may be either completely closed, or (as represented in the diagram) perforated by a larger or smaller opening. Behind the cornea is a chamber known as the anterior optic chamber. This chamber is continued back on each side round a great part of the circumference of the eye, and separates the sclerotic from a layer internal to it.