EYE, COMPARATIVE ANATOMY OF THE. How far down in the scale it is necessary to go to fine., animals which have no eyes depends upon wheth er we mean by 'eye' an organ capable of form ing optical images, or simply an organ capable of responding to the stimulus of light. If the latter is really an eye, then we are justified in saying that all the large groups of animals higher than sponges have eyes, though many families and even some orders may lack them. Among ecelen terates, simple light-detecting organs, known as pigment spots, or 'pigment eyes,' and consist ing of groups of pigment-cells associated with sensory cells, occur in many and in ctenophores. In some cases the cuticle over these spots is specially thickened to form a sort of a lens. Similar pigment eyes occur among the flat worms (Platoda). and in some eases they are somewhat more complicated by the additions of so-called retinal cells with rod-like processes. It is very doubtful, however, whether these eyes are really anything more than light-detecting organs. Pigment eyes very similar to these, though often somewhat more complicated, occur in many Torras, crustaceans, insects, and mol lusks, and in a few echinoderms. The eyes of tnnieates and of Amphioxus are not of any higher degree of organization. In crustaceans the pig ment eye is known as the 'unpaired' eye, and is apparently made up of three simple eyes fused together. In insects the pigment eyes are called %coin,' and these also occur in spiders and scor pions. Pigment eyes are of service to their pos sessors in enabling them to distingnish between light and shade.- and in detecting different degrees of light. Thus shadows east by an approaching enemy would be noticed, and, in aquatic animals.
tapprmeli to the surface would be quickly indi cated.
Turnimf now to those organs which unques tionabl• form some sort of an image, we find there are two very distinct kinds—'simple' and 'compound' eyes. Simple eyes correspond iu structure to a greater or less degree with the eye of man, while compound eyes are of a totally dif ferent kind. The latter are found only among arthropods, where they reach a high degree of development, the so-called compound eyes of sonic mollusks and sea.urchins being really much less complex. In arthropods the optic nerve bears two noticeable swellings—the optic ganglion, really a part of the brain, and the retinal gang lion. from which radiates the nerve fibres, enter ing the retinal cells. The retinal cells are grouped in clusters of four to seven, known as which are more or less heavily pigmented distally. Each retinula is the basal part of a single eye, the upper portion of which consists of a crystal cone (wanting in the eyes of many insects), and of hypodermal elements covered with the chiti nous cuticle developed as a cornea. These single eyes are crowded together, though separated from each other by pigment-cells, on a strongly convex basal membrane, thus forming a more or less convex compound eye. Rays of light falling on the eye are absorbed without giving rise to a vis ual stimulus, except such as are directly parallel to the long axis of the single eyes. Each of these eyes therefore forms an image of that which lies directly before it, and the whole compound eye thus forms a mosaic, probably with sharp out lines, but wholly lacking perspective.