THE STRUCTURE OF THE EYES IN THE LOWER ANIMALS Already in the Protozoa, however, true eyes are found. In Vol vox and other Flagellates they consist of a lens, and, behind this, an accumulation of the plasma, which frequently is pigmented. The earthworm has isolated pho to-sensitive cells, which may be distinguished by their structure; the simplest of the vertebrates (Branchiostoma) has numerous isolated photo-sensitive cells scat tered over the whole of the dor sal nerve cord, each of which is half surrounded by a pigment cell (fig. I).
Eyes of the most simple kind originate through the concentration of a number of such photo sensitive cells in a small space.
are found in Medusae, starfishes and some Annelid worms (fig. 2). The first step in the perfecting of this very primitive apparatus is the sinking of the eye-spot into a pit-like depression, thus forming an eye-cup (optic cup). This type already, perhaps, is adapted for the perception of the direc tion of light, and is of very general occurrence, being found in some worms, gastropods, bivalves, starfishes, Arthropods, etc. (fig. 3). This type of eye when it occurs in Arthropods is called an ocellus, and is more complicated, as the thin cuticle, which envelopes the whole of the body, becomes thickened over it, and forms a strongly arched, or occasionally, ball-shaped lens. At the same time division of labour takes place among the cells forming the eye-cup. The sides of this remain transparent, and secrete a kind of vitreous humour, only the cells situated at the back of the cup forming the retina (fig. 4a). In yet other cases two layers may be formed in the optic cup by a peculiar process of folding, which cannot be described here. A vitreous outer layer and an inner retinal layer can then be distinguished (fig. 4b). The open cup-shaped eye may become quite closed in, forming an optic vesicle, by the growing together of the outer margins (as in many Chaetopod worms, gas teropods and cuttlefishes). The interior of this optic vesicle is then filled by a jelly-like sub stance, the vitreous humour. The anterior portion of the vesicle is transparent, like the skin which grows over it ; the inner half becomes the retina (fig. 5). This
type of eye is found in its most perfect form in some worms (Alciope) and gasteropods (Helix, Limax) with the formation of a lens, which is free in the vitreous humour, and condenses the rays of light. The cuttlefish alone shows a further development. This is the highest of all inverte brate animals. In general, as well as in psychical, development, it ranks at least as high as fishes.
The eye represented in fig. 6, presents a striking resemblance to that of a vertebrate. All the separate parts of the one are repeated in the other. The posterior chamber is the principal part; it is bounded at the back by the retina, and in front by the iris. It is filled by a jelly—the vitreous humour.
The lens is spherical, as in fishes. It differs from the lens of a fish, however, in that it does not consist of cells; it is divided by a delicate skin which separates the anterior chamber of the eye from the posterior, half lying to the front, and half to the back.
The anterior chamber is closed by a transparent cornea, in front of which are movable eyelids. The power of accommoda closely those cut on a precious stone. Each facet corresponds to a single eye or ommatidium, which always takes the shape of an elongated cone. In each of these eyes, taken by itself, the follow ing parts may be distinguished. The outermost part consists of a small, somewhat rounded, transparent lens. Under this is the tion is exactly the same as in the vertebrate eye. The cuttlefish can focus its sight either for near vision or for distant by means of particular muscles, which alter the length of the axis of the eye. It is hardly necessary to mention that the pupil in the iris can be expanded or contracted according to the strength of the light falling on it. In the structure of the retina, also, the eye of the cuttlefish is equivalent to those of vertebrates.
It is made up of more than ioo,000 elements per sq.mm., and thus is certainly adapted for the formation of a sharp image. Lastly, it may be mentioned, that in size also it is in no way