5.--CLOSED, VESICULAR EYES of- (A) A MAKINt VAbl rwrvu (MUREX) ; (B) A LAND GASTROPOD SLUG (LIMAX) crystalline cone, a conspicuous structure, usually gelatinous, which is quite transparent, and allows the rays of light to pass freely, and directs their path. If we examine the eye of a diurnal insect, we find that the inner extremity of the crystalline cone is adjacent to the cells which, collectively, correspond to our retina. Each ommatidium has only 6 or 7 of these cells. Among them they se crete a remarkable body, the rhabdom, which evidently repre sents a kind of transformer.
The rays of light which enter the eye are focused on the rhab dom, and there are evidently brought into such a form of energy that they are available to the sensory cells. The nervous layer immediately adjoins the sensory cells. In diurnal insects each ommatidium is separated from its neighbour by a layer of pigment.
Somewhat comprehensive conclusions may be drawn from the structure of the faceted eye as to its functional powers. It seems certain that each ommatidium functions separately. It inferior to the vertebrate eye. Eyes 37 cm. in diameter have been found in the giant cuttlefishes of the genus Architeuthis.
The eyes of turbellarian worms and leeches have a somewhat different structure. In these animals, it is true, eye-cups are present, but they are not formed by the retinal cells themselves, but of pigment cells, which absorb the light, while the sensory cells penetrate into the interior of the cup (fig. 7).
a matter of course that only in a very slight degree is an insect Eyes of Nocturnal Insects.—The eyes of nocturnal insects and of the mostly nocturnal crustaceans are constructed in a remarkably different manner from those of diurnal insects. As is to be expected, such an eye is constructed so as to admit •a greater amount of light than that of a diurnal insect. This is attained by the ommatidia being no longer separated by pigment layers. In addition, the crystalline cone has, in such animals, other physical peculiarities. The physiologist Exner has proved this conclusively for the eyes of the glow-worm. He showed that the light given out by a luminous point penetrates not only one ommatidium but many (fig. 9). After leaving the crystalline cone, however, the rays are refracted in such a way that, ulti mately, they again become focused upon one point of the retina. Each rhabdom receives, therefore, a much greater amount of light than those in the eyes of diurnal insects. On account of the arrangement one upon the other of the points of light which come from the various crystalline cones, such eyes are described as giving "superposition images," while those of diurnal insects give "apposition images." It is of interest that crustaceans are able to use their eyes either for superposition or for apposition vision, according to their needs. This is attained by the move ment of the pigment in the cells which separate the ommatidia.
able to distinguish form, as is borne out by the previously men tioned experiments. Most insects, which have only a few thou sands or even hundreds of ommatidia, cannot do this at all. On the other hand, these eyes are eminently adapted for the per ception of movement. When an insect the size of a fly passes in front of a dragonfly at a distance of about 2 metres, it is seen by the latter only as a black point, for it can be proved that, in this case, only one single ommatidium catches the image of the fly. In spite of this it is pursued, captured and eaten.
Faceted eyes are found in some bivalves (Arca) and Chaetopod worms, as well as in Arthropods, but serve, however, only for perception of movement (fig. 1o).