Ocelli of Pecten.—In Pecten and Spondylus the ocelli are remarkably large and prominent, shining like precious stones, and are placed along the two edges of the mantle so as to receive the light when the shell gapes (Fig. [93]). In Pecten opercularis, jacobaeus, and maximus their number varies from 80 to 120. In Spondylus gaederopus, a very inequivalve shell, 60 have been counted on the right or fixed valve, and 90 on the left or upper valve. Each ocellus is connected, by means of its optic nerve, with the large circumpalleal nerve, and so with the branchial ganglion. They possess a cornea, lens, choroidea, and optic nerve, and, according to Hickson,[298] bear a considerable resemblance to the vertebrate type of eye. In spite of this, the power of vision in these genera does not appear at all superior to that of other Pelecypoda.
Fig. 93.—Pecten opercularis L., showing the ocelli on the two edges of the mantle.
Fig. 94.—Compound eyes (c.e) of Arca barbata L.; m.l, mantle fold; omm, ommatidia. (After Patten.)
According to the elaborate investigations of Patten, the ‘eyes’ in Arca occur upon the middle or ‘ophthalmic’ fold of the mantle-edge, which is thickened at the end to admit of their reception. Along this is ranged a row of dark brown spots of various sizes, which are larger at the anterior and posterior ends of the mantle-edge, but smaller and more numerous towards the middle. These brown spots, or ‘eyes,’ are many of them compound, being made up of the fusion of a number of ommatidia (from 10 to 80) into one large round eye, which is generally elevated above the surface of the surrounding epithelium. Sometimes these eyes themselves tend to fuse together. In one specimen of Arca Noae, 133 of these faceted eyes were counted in one mantle border, and 102 in the other.
There can be little doubt that the development of these functional eyes, or sensitive spots, in bivalve Mollusca, is due to special needs. They appear to be entirely absent in fresh-water bivalves (with the exception of Dreissensia, which is obviously a marine genus recently become fresh-water), while they are most abundant in genera living between tide marks (Solen, Mya, Mactra), and most highly specialised in a genus that is, for a bivalve, of singularly active habits (Pecten). Now genera living in sand between tide marks, as the three above-mentioned genera are in the habit of doing, and also protruding their siphons, and occasionally a considerable portion of their shells, out of their burrow, are manifestly very much at the mercy of their watchful enemies the gulls, and anything which would enable them to apprehend the approach of their enemies would be greatly to their advantage. Here, perhaps, lies the explanation of the greater elaboration of these pigmented spots in littoral genera, as compared with those inhabiting deeper water. Pecten, again, a genus distinguished by great activity, which can ‘fly’ for considerable distances in the water by flapping its valves together and expelling the water from the apertures at either side of the hinge, may be greatly assisted by its ocelli in directing its flight so as to escape its enemies.
III. Smell
The sense of smell—touch at a distance, as Moquin-Tandon has called it—is probably the most important sense which the Mollusca possess, and is unquestionably far more valuable to them than that of sight. Any one who has ever enjoyed the fun of hauling up lobster pots will recollect that part of the contents was generally a plentiful sprinkling of Buccinum, Nassa, and Natica, attracted by the smell of the stinking piece of fish with which the trap was baited. According to Mr. J. S. Gibbons,[299] Bullia rhodostoma congregates in hundreds on gigantic medusae which are stranded on the sandy bays near the Cape of Good Hope. Dr. J. G. Jeffreys says[300] that quantities of the common Neptunea antiqua “are procured on the Cheshire coast by the fishermen placing a dead dog on the sands at low-water mark during spring tides. The bait is then completely covered with stones, which are piled up like a cairn. On the next turn of the tide the heap of stones is visited, and the whelks are found on the surface in great numbers, having been apparently attracted by the smell of the bait, but unable to get at it.” Mr. W. A. Lloyd kept specimens of Nassa reticulata in a tank in the sand, at the bottom of which they usually remained buried. If a piece of meat of any kind were drawn over the sand, the Nassa would appear above the surface in a few minutes. Half-picked beef or mutton bones, if placed in the tank, were covered in a few minutes. In fact, no animal matter, whether living or dead, could be introduced without the Nassa smelling it, and coming up to see what they could get.[301]
Any one can experiment for themselves on the olfactory powers of our common snails or slugs. Moquin-Tandon records[302] two interesting cases, one communicated to him by letter, the other occurring to himself. His correspondent, a M. Parenteau, was one day walking along a dusty high-road, when he noticed, near the middle of the road, an empty bean-pod and two Arions eating it. Attributing the meeting of feeders and food to mere chance, he was walking on, when he noticed a second bean-pod, and, about two yards away from it, a third Arion, hurrying straight towards it. When the Arion had yet more than a yard to traverse, M. Parenteau picked up the bean and put it in his pocket. The Arion stopped, raised its head, and turned in every direction, waving its tentacles, but without advancing. M. Parenteau then carried the bean to the other side of the road, and put it in a small hole behind a piece of stone. The Arion, after a moment’s indecision, started off straight for the bean. Again the position of the precious morsel was changed, and again the Arion made for it, this time without being further tantalised. M. Moquin-Tandon noticed, one rainy day in the botanical gardens at Toulouse, two Limax maximus approaching a rotten apple from different directions. He changed the position of the apple several times, placing it at a sufficient distance, to be sure they could not see it, but they always hit it off correctly, after raising their heads and moving their long tentacles in every direction. It then occurred to him to hold the apple in the air, some centimetres above the head of the Limax. They perceived where it was, raised their heads and lengthened their necks, endeavouring to find some solid body on which to climb to their food.