Onchidium (a genus of slugs, widely spread over the Southern Hemisphere), in which Semper has shown* that the nerve actually pierces the retina as in verte

Fig. 97.-Diagram of eye of Pecten (after Hickson). a, Cornea; b, transparent basement membrane supporting the epithelial cells of cornea; c, the pigmented epithelium; d, the lining epithelium of the mantle; e, the lens; f, the ligament supporting the lens; g, the retina; h, the tapetum; k, the pigment; m, the retinal nerve; n, complementary nerve.

brates. That this distinctive character should thus reappear in so distant a group is very interesting, and it is also remarkable that Onchidium possesses two kinds of eyes: some on the head, which are constructed on the same type as those of other molluscs; while the peculiar eyes just mentioned are scattered over the back, and their nerves arise, not from the cephalic, but from the visceral ganglion. Moreover, they differ in number, not only in the different species, some having one hundred, some as few as twelve, and others none at all, but even in different individuals of the same species. Indeed, they are continually growing and being reabsorbed. But while thus resembling a simple vertebrate eye, the dorsal eyes of Onchidium have a totally

* "Ueber Schnecken Augen am Wirbelthier typus," Arch. für Mic. Anat., 1877,

different development, arising, except the nerve, entirely from the integument; on the contrary, in the vertebrate eye, while the cornea and lens are formed from the skin, the retina is an outgrowth from the brain.

Semper does not suppose that the Onchidia perceive any actual image with their dorsal eyes, and thinks that they are merely able to distinguish differences in the amount of light.

They are shore-living molluscs, and are preyed on by small fishes belonging to the genus Perophthalmus, which has the curious habit of leaving the water and walking about on the sand in search of food. The back of the Onchidium contains a number of glands, each opening by a minute pore; and Semper suggests that, when warned by the shadow of the fish, the little slugs eject a shower of spray, drive off their enemy, and save themselves. This is not quite so far-fetched as might at first sight appear, for we know that there are many other animals, the sepia, many ants, the bombardier and other beetles, etc., which defend themselves in a similar manner.

It seems difficult to understand why the Onchidia should be endowed with so many eyes. The irrelative repetition of organs meets us, however, continually in the lower animals. Moreover, in the present case Semper has thrown out a plausible suggestion. The organs of touch (see ante, p. 14) curiously resemble eyes in structure, and a very slight change might make them capable of perceiving light. It is possible, then, that some of them may undergo a change of function, and that this may throw some light on the variability in number.

In the Chitonidæ, where dorsal eyes have recently

SENSE-ORGANS OF CHITON.

145

been discovered by Moseley,* they are even more numerous. Chiton itself, indeed, has none; but in Schizochiton there are 300, and in Corephium more

Fig. 98. Schematic representation of the soft and some of the hard parts in a shell of a Chiton (Acanthopleura), as seen in a section vertical to the surface, and with the margin of the shell lying in the direction of the left side of the drawing. a, Conical termination of sense-organ; b, b', ends of nerve; c, nerve; f, calcareous cornea; g, lens; h, iris; k, pigmented capsule of eye; m, body of sense-organ cut across; n, nerve of eye; p, nerve of sense-organ; r, rods of retina.

than ten thousand. As in Onchidium, they probably arose as modifications of the organs of touch, and are supplied by the same nerves. They possess (1) a cornea, (2) a perfectly transparent and strongly biconvex lens, and (3) the retina, which presents a layer of short but well-defined rods. It is interesting that they point towards the light, and not, as in Onchidium, away from it.

"On the Presence of Eyes in Shells of certain Chitonidæ," Quarterly Journal of Microscopal Science, 1885.

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EYES OF CRUSTACEA AND INSECTS.

CHAPTER VII.

THE ORGANS OF VISION IN INSECTS AND CRUSTACEA.

I NOW pass on to the eyes of insects. In most species of this group there are two distinct kinds: the large compound eyes, which are situated one on each side of the head; and the ocelli, or small eyes, of which there are generally three, arranged in a triangle, between the other two.

Speaking roughly, the ocelli of insects may be said to see as our eyes do; that is to say, the lens throws on the retina an image, which is perceived by the fine terminations of the optic nerve. One type of such an eye in a young water-beetle (Dytiscus) is shown in Fig. 84, p. 131. This illustrates the mode of development of an ocellus, which has been already referred to (ante, p. 131).

The structure of fully formed ocelli is shown by Fig. 99. In details, indeed, they present many differences, and it is remarkable that in some species this is the case even with those of the same individual; for instance, in those of one of our large spiders, Epeira diadema (Fig. 99).

In this case the eye B would receive more light, and the image, therefore, would be brighter; but, on

the other hand, the image would be pictured in greater

detail by the eye A.

6

Fig. 99.-Long section through the front (4) and hinder (B) dorsal eyes of Epeira diadema (after Grenacher). A, Anterior eye; B, posterior eye; Hp, hypoderm; Ct, cuticle; ct, boundary membrane; K, nuclei of the cells of the retina; M, muscular fibres; M, M', cross sections of ditto; St, rods; Pg, P', pigment cells; L, lens; Gk', vitreous body; Kt, crystalline cones; Rt, retina; Nop, optic nerve.

Speaking generally, an ocellus may be regarded as consisting of

1. A lens, forming part of the general body covering. 2. A layer of transparent cells.

3. A retina, or second layer of deeper lying cells, each of which bears a rod in front, while their inner ends pass into the filaments of the optic nerve.

4. The pigment.

From the convexity of the lens it would have a short focus, and the comparatively small number of rods would give but a very imperfect image, except of very near objects.

But though these eyes agree so far with ours, there is an essential difference between them. It will be at