Fig. 3.-Illustration of the change in the form of the lens when adjusted-a to distant, b to near objects.

Sommering is said not to exist in any mammals below the quadrumana; the tear gland is found in all except cetacea. In birds the sclerotic becomes more or less strengthened by cartilage, and in the neighborhood of the cornea is provided with a series of bony plates, arranged in a circle, and overlapping each other; but the chief peculiarity consists in the pecten, folded like a comb or fan, and projected forward toward the lens; it is vascular like the choroid, though not connected with it, and is dark with pigment; its use is not satisfactorily ascertained. Many species of reptiles have osseous pieces in the sclerotic; snakes have no movable lids; the chameleon has a single circular lid. In fishes the eyes are generally large, the sclerotic thick, and in some (as the tunny) osseous anteriorly; they have neither lids, except the most rudimentary, nor lachrymal glands; the cornea is very flat, and the lens dense; around the entrance of the optic nerve there is a very vascular, horseshoe-shaped organ, between the layers of the choroid, called the choroid gland or muscle.

The organs of vision in insects consist of simple or of compound eyes, the former occurring chiefly in larvae, the latter in perfect insects; they are wholly absent in some larvae, and both forms coexist in the perfect state of many. The simple eyes {ocelli or stemmata) consist of a convex cornea, behind which is a lens, lodged in an expansion of the optic nerve, and surrounded by a variously colored pigment layer; they vary in number from two to more than 100, and are situated on the head. The compound eyes are made up of simple eyes so closely placed that their facets or corneas are contiguous; behind each cornea is a transparent pyramid whose interior apex is received into a kind of vitreous body, surrounded by the nerve and the choroid; there are sometimes many thousand facets in these eyes, which may cover nearly the whole head, and hairs may project at their angles. In the arachnids the eyes are simple, and the orders have been characterized by their number, situation, and direction; they are most numerous in the scorpions.

The sense of sight is present in almost all Crustacea; their simple eyes consist of a cornea with a lens and pigment layer; a usual form is that of many simple eyes, placed close together, and covered by a common cornea; sometimes there is a faceted cornea under the simple one; the highest forms have compound faceted eyes, in many situated at or near the end of two peduncles movably articulated to the cephalo-thorax and concealed in special fossae; these facets are very numerous, and behind each is the usual lens and pigment. The eyes of cephalopods are very large and highly developed, resembling in some respects the vertebrate organ; there is generally an ocular bulb, and a capsule constituted by a cartilaginous orbit and a fibrous continuation of the cutaneous envelope, which takes the place of a cornea; semi-lunar folds containing muscular fibres cover the eye like lids; in front of the globe is a space analogous to an anterior chamber, containing a serous fluid, and in the octopods communicating externally; internally this chamber is closed by a kind of pupil; its serous membrane has a silvery lustre; in some species the lens is in direct contact with the water in which they swim; there is an iris, sclerotic, vitreous liquid, a spherical brownish lens formed of concentric layers, a ciliary body, and pigment layer; in the nautilus the eyes are placed on a projecting stalk, but in others are generally deeply sunk in the head.

In the cephalophora (including pteropoda, heteropoda, and gasteropo-dous mollusks) eyes are generally present, never more than two in number and comparatively small; they are almost always connected with the tentacles, either at their base, sides, or extremities. In acephalous mollusks eyes are very common and numerous, occupying the borders of the mantle or confined to the orifices of the tubes, and are either pedunculated or sessile. In the annelids the eyes are generally either wanting entirely, or are merely able to distinguish light from darkness; but the leeches have from two to ten undoubted eyes. In the helminths there appear to be no eyes, only pigment spots containing no light-refracting body. Below these are found in the radiata various eye specks and pigment dots which doubtless in some cases are true eyes, but authors are not yet agreed as to the light-refracting powers of most of these organs. The eye of the blind fish of the Mammoth cave, Kentucky, though unable to form a distinct image, can doubtless distinguish light from darkness through the areolar tissue and skin which cover it; Prof. J. Wyman has found in it a lens, sclerotic, choroid, retina, and optic nerve, and it is therefore constructed on the vertebrate plan, rather than the invertebrate to which it has generally been compared; the parts in connection with the nervous system are developed, while those which are formed by inversion of the integuments are mostly absent; some authors are of opinion that the stimulus of light for several generations would retransform this eye into an ordinary organ of vision.