Although it is probably not functional in any existing form, mention must here be made of the median or pineal eye. On the head of the common slow-worm, or blind-worm, there is a dark patch surrounding a brighter spot. This is the remnant of a median eye. It has been found in varying states of degeneration in many reptiles (Fig. 34), and in a yet more vestigial form in some fishes and amphibia. It is connected with a curious structure, associated with the brain of all vertebrates, and called the pineal gland. Descartes thought that this was the seat of the soul; but modern investigation shows it to be a structure which has resulted from the degeneration of that part of the brain which was connected with the median eye. There is some reason to suppose that, in ancient life-forms, like the Ichthyosaurus, and Plesiosaurus, and the Labyrinthodont amphibians, it was large and functional. In any case, there is a large hole in the skull (Fig. 35) through which the nervous connection with the brain may have been established. The structure of the eye is not similar to that of the lateral eye, but more like that of some of the invertebrates. To these invertebrates we must now turn. [Illustration: Fig. 35.--Skull of _Melanerpeton_. A Labyrinthodont amphibian from the Permian of Bohemia (after Fritsch). × 4. Pa., the parietal foramen.] * * * * * Insects have eyes of two kinds. If we examine with a lens the head of a bee, we shall see, on either side, the large compound or facetted eye; but in addition to these there is on the forehead or vertex a triangle of three small, bright, simple eyes, or ocelli. These ocelli, or eyelets, differ, in different insects, as to the details of their structure; but in general they consist of a lens produced by the thickening of the integumentary layer which is at the same time rendered transparent. Behind this lies the so-called vitreous body, composed of transparent cells, and then follows the retina, in which there are a number of rods, the recipient ends of which are turned towards the rays of light, and not away from them as in the vertebrate. Spiders have from six to eight ocelli, arranged in a pattern on the top of the head. Facetted eyes are not found in them. [Illustration: Fig. 36.--Eyes and eyelets of bee. A. Drone. B. Worker.]