SLUGS are regarded by most people with abhorrence; they seem repulsive to the touch and sight, are regarded as de-spoilers of the garden, of little use in any way and fit only to be trodden underfoot or otherwise destroyed at the very
earliest opportunity. In a few outlying districts, it is true, a particular type of slug boiled in milk is still regarded as a sovereign cure for consumption, but even those who recommend this cure can easily be made to admit that they regard the supposed cure as little better than the disease itself. Nevertheless, as is usually the case with supposedly noxious animals, there is something to be said in favour of some slugs at least, and indiscriminate killing by gardeners is not the best method of dealing with them.
The carnivorous slugs, however unattractive their appearance may be, are among those with some claim to virtue in human eyes. To begin with, they do not eat vegetable matter except when it has reached an advanced stage of decomposition, and in this they may be classed as effective scavengers, especially as they will devour garbage and scraps of all kinds, vegetable or animal, including dead bodies of small animals, insects, or even the carcases of their own kin. Other slugs feed largely on toadstools and help to keep the countryside clear of the decomposing remains of these fungi which are often obnoxious. One of them, about three inches long and readily recognisable because the hinder end of its body is broader than any other part and bears a small shell, not only feeds on worms, actually pursuing them into their burrows, but will even feed on other slugs and snails.
It not infrequently happens that the estimable qualities of the carnivorous snails become perverted. In this case their predatory and scavenging tendencies work contrary to human desires and needs. Some take to feeding on paper, with dire results if they become established in the house of a bibliophil. Let loose in a dairy, they will feed on cream or butter or whatever else may be available; and in a larder will do similar damage, having even been known to attack the family joint.
A MOUTHFUL OF TROUBLE: THE SLUG WITH FIFTEEN THOUSAND TEETH SLUGS capable of hunting and killing worms would naturally be expected to be possessed of specially constructed jaws or some other apparatus for seizing and holding so wriggling a prey, but actually the mouth of a carnivorous slug differs little from that of other Gastropods, and not at all in the principle of its construction. In Gastropods there is a small horny plate which is dignified by the name of jaw, but this
only serves to cut off the food. More rarely two such plates are present. The process of mastication is performed by the tongue, or more properly speaking, the radula. This consists of a thin ribbon of a horny substance known as chitin and is covered with numerous teeth of the same material. The pattern, size and ornamentation of the radula vary enormously in different species. In the sea-slugs, for example, there are often few more than a dozen such teeth on the radula, but in the garden slugs there may be nearly fifteen thousand set in rows of about a hundred each, the whole looking under microscope like a very irregular file. The action of using the radula is precisely a filing motion, the food being rasped up into pieces sufficiently small for the stomach to deal with.
In the cuttlefish there are a pair of horny jaws and a radula, but the latter is reduced to extremely small dimensions whereas the jaws are much enlarged and strengthened and curved to form a formidable beak, like a parrot’s beak in shape and having similar powers. The Lamellibranchs (oysters, mussels, cockles, etc.), on the other hand, have neither jaws nor radula and the head has become practically nothing more than an opening into the gullet, for the simple reason that they feed on small plants and animals of microscopic dimensions carried in by the same currents of water that bring oxygen to their gills. In this case the food is trapped on the gill-plates and carried up to the mouth by rows of fine protoplasmic hairs.
The feeding mechanism of the Lamellibranchs is not at all easy to describe in a few words, but by dealing in round terms with the anatomy of the oyster, it may be possible to make clear the general principles on which it works. An oyster is roughly comparable to a book of which the valves form the covers. Immediately within the valves, and actually responsible for their formation and growth, are two broad flaps of flesh known as the mantle. Within the mantle are the gills, delicate plate-like organs composed of a fine lattice-work plentifully supplied with blood-vessels and covered with countless fine protoplasmic hairs. The gills constitute what is known popularly as the ‘beard ‘. At the centre, between the pair of gill-plates on either side, is the muscular foot containing the vital organs. In life the protoplasmic hairs covering the gills are in constant and concerted action, driving a current of water across the gill-plates, the edges of the mantle being so arranged that water can be drawn in at one point and driven
out at another. In many bivalves the process is assisted by siphons, or tubular prolongations of the edge of the mantle, one siphon for drawing the water in and the other serving as a waste-pipe.