ALTHOUGH in general construction, in the possession of highly organised digestive and respiratory systems, and in their habits, the Mollusca show a considerable advance on the backboneless animals or invertebrates considered so far, the development of their nervous system does not appear to have kept pace with the development of the other parts of the body. They have nothing comparable with the highly organised and centralised brain of the vertebrates. Their nervous system consists of a series of nerves with various ganglia, or masses of nerve cells, at the junctions of the main nerves. Certainly many of them, as for example, oysters, show very little sign of intelligence—little more, in fact, than the more primitive animals, such as sea-anemones and jellyfish. But it would be wrong to brand the whole group on the attainments of a few of its members. Some show quite a high degree in the development of the senses, and the cuttlefish, as will be shown later on, compare favourably in this respect with some of the lower vertebrates.
In oysters and many others the sense of sight is feeble and the organs doing duty for eyes are little more than shallow pits in the edge of the mantle, filled with pigment. Slugs and snails, on the other hand, have well-developed eyes, as everybody knows, since from early childhood we must all be accustomed to seeing these animals draw in their stalked eyes when touched, and protrude them again after we have left them alone for a short while. But even slugs and snails have a very limited field of vision, extending to little more than a fifth of an inch. The use of their eyes is rather for the perception of moving shadows, warning them of the vicinity of a potential enemy. In cuttlefish, however, the eye has a lens and retina and is undoubtedly a very efficient organ of sight, a strict necessity in animals depending as they do on the hunting of swiftly moving prey.
The sense of hearing is very slight in all molluscs, although they are capable of appreciating sound, as experiment has shown. But apart from the cuttlefish, it is undoubtedly on the sense of smell and touch that they depend for their food and for escape from enemies. Whelks are caught by baiting them with rotting fish; slugs and snails will smell out an apple or a dish of cream and make their way directly to them, showing that they have smelt them from a distance; and even the supposedly stupid oyster will close its valves on the approach of its arch-enemy, the starfish. Experiments have shown that a little of the juice of a starfish placed in the water near an oyster will cause it to close. That the sense of touch is acute needs little proof, as anyone who has tried to insert a small pebble into the gaping valves of a mussel or has touched a snail and seen it withdraw into its shell will testify. Even a very slight touch on the shell of a snail is instantly communicated to the animal’s nervous system and is sufficient to make the snail withdraw.
YOU WILL FIND THE SNAIL AT HIS PERMANENT ADDRESS IT WOULD hardly be proper to leave the question of the senses of Mollusca without saying something of the homing instinct, not uncommon among animals of all types, but particularly striking in certain molluscs, perhaps because it is least suspected there. The most beautiful example is seen in the limpets. The young limpet settles down on a spot on a rock, and that spot remains its home for the rest of its life. As its shell grows, the edge takes on the contour of the surface of the rock and fits exactly to it, with the result that it is difficult to get anything underneath to prise the limpet off. A limpet feeds on seaweeds and often travels quite considerable distances in search of food, yet, unless some accident overtakes it, it will always return to its ‘home.’ Moreover, it always orientates itself in such a way that it takes up precisely the position it had before embarking on its excursion, with the result that it fits on to the rock, as closely as though it had never left it.
With most other molluscs there is not such a need for this extreme precision, but even they have their preferences for particular corners in which to lie up when resting. Snails and slugs will often travel long distances in a single night and return before dawn to the spot they left. Experiments have been made to test the truth of this. Snails have been taken from certain crannies in a wall, their shells have been marked with distinctive dabs of paint and the animals then hurled for yards, often over a garden fence or a wall. Sure enough, the next morning they have been found again in the same places from which they were taken the night before. Again and again this has been done with the same snails and just as persistently they have returned except when they have suffered injury in falling after being thrown, or disaster, perhaps in the shape of a thrush, has overtaken them.