Our British Snails

by J. W. Horsley

Body of snail and of slug.

Without going into details of classification and anatomy, which would only deter or puzzle a beginner, let me take two typical molluscs of those which we shall find in England, the common garden snail Helix aspersa, and a freshwater mussel, Unio margaritifer, and see where they come in the scale of creation and what are their powers and peculiarities.

Molluscs (mollis esca, soft food—boneless creatures) are below the aristocracy of the vertebrates or backboned creatures, and so they come just below the Fishes, but above the Insects. They are divided into those possessing a head and those possessing no head (although with some sort of a brain or organ of sense), the snail being of the former class and the mussel of the latter. The former are univalves and the latter bivalves having two shells for protection. The latter also are restricted to life in water, whereas the former are found both on land and in water, e.g. the snail and the whelk, although for ages probably no molluscs were air-breathing land dwellers. In the class of Cephala, to which our snail belongs, there is the sub-class of Gasteropoda, or stomach-footed, because on the ventral side of the body a sole-like disc or foot exists, by the wave-like expansions and contractions of which the animal progresses.

In this sub-class there is a division according to their having or not having an operculum, or means of closing and protecting the orifice of the shell. Most gasteropods which live in water have this; most which live on land (only two exceptions in British molluscs) have not. Here again we must trace our snail down to the sub-order of Pulmonata, or lung or air-sac breathers as distinct from its sisters which inhabit water and breathe by gills. This sub-order is again divided into various families, Arion, Limax, Testacella, Vitrina, Zonites, Helix, etc., and Helix again is divided into various genera, of which Helix is one, and even this is subdivided into sub-genera, Patula, Punctum, Acanthinula, Vallonia, Chilotrema, Gonostoma, Pomatia, Tachea, etc., and to the sub-genus Pomatia our garden snail as well as the “Roman snail” belongs. Looking backwards we, therefore, place our friend as the species aspersa, of the sub-genus Pomatia, of the genus Helix, of the family Helicidæ, of the sub-order Pulmonata, of the order Inoperculata, of the sub-class Gasteropoda, of the class Cephala, of the sub-kingdom of Mollusca, of the kingdom Invertebrata or backboneless animals.

It belongs by origin not to the earliest form of snail, but to the most highly organized group in the world, especially characteristic of the European region, and possessing in their superiority the power to colonize and dispossess the original native snails of other lands. The shell is globular in form with five whorls (the Greek word “helix” means a coil), each usually marked with five bands of pigment. It is mainly a vegetarian, and by habit a lover of the twilight and of moisture. With the exception of H. pomatia it is the largest of our native shells, and is too common to satisfy gardeners. A powerful animal of its kind, it can travel a yard in twelve minutes, or at the rate of a mile in a fortnight, can bear or draw on level ground a weight fifty times its own. It breathes about four times a minute, and its heart-beat varies from sixty to eighty per minute according to temperature, or its activity. It takes its winter rest in clusters, closing its mouth with a membranous film, while if the cold increases it shrinks farther into its shell and makes more epiphragms or film curtains to keep out the cold. Not only on the Continent, but in several parts of England, notably about Bath and Bristol, it is sought, sold, and used for food, and in Belgium it is said to be preferred to the larger and more firm-fleshed H. pomatia. The eggs, from forty to a hundred, are laid in the earth and hatched in from a fortnight to a month, according to the weather. I had observed them as a boy, and used to call tapioca pudding “snail’s egg pudding.” In the year of their hatching they attain but half their proper size, but after hibernation they eat voraciously and grow rapidly, so as to attain full size in a little more than a year. Most die in their second hibernation (if not destroyed by their many enemies, gardeners, collectors, rats, rabbits, ducks, thrushes, and beetles); but when kept and protected for observation they have achieved the great age of even ten years.

They have a great power of “homing” like pigeons, however far (for them) is their journey after favourite food. The slime-marked journeys or feeding tracks of this species (and still more of slugs) afford matter of great interest. As to sight the two eyes are the dark specks on the tip of the upper pair of “horns,” but the range of vision is very short indeed, and the difference between light and approaching darkness is all that some seem able to perceive. The organs of hearing are two small sacs filled with fluid in which are some calcareous grains. They hear little which is audible to human ears, and if not altogether deaf they are dumb as far as we can hear. The power of taste they possess, as is shown by the preference of some foods to others. The sense of touch is acute and resides in all parts of the soft and moist external skin, and especially in the upper tentacles or horns in the Helicidæ. Jaws they have with which to seize and to bite off food, and in H. aspersa and others these bear teeth, but the chief work is done by a sort of toothed tongue, the radula, which rasps off particles of food with a side to side motion of the head as the animal advances. Our aspersa has 12,615 teeth on this ribbon, contained in 145 transverse rows. The organs of digestion are complex and practically much the same as our own. Little vegetation would be left in nature had not, on the one hand, snails been kept down by many enemies as well as by their need of hibernation and their short life; while on the other by numerous devices in the course of ages many plants have protected themselves against the moving machine of a snail’s mouth. Cultivated plants, which generally lose their natural protections, have to be guarded by human guards or gardeners. Some plants defend themselves by prickles or hairs, some by hardening themselves with lime or flint, some by bitter or acrid juices. A heart of two chambers, veins, arteries, and blood our snail possesses, and, like man, the old snail has a slower pulse than the young one, and in both exercise increases the pulse rate and also warmth. Breathing is accomplished by a single chamber or air-cell, but also through the skin. As in the case of plants, some kinds are male and female separately, and as some have both powers and products in the same plant, so also is it with mollusca. H. aspersa and most Gasteropoda are of the latter kind.

Having now taken H. aspersa as the representative of our univalves, let us take the “Pearl Mussel”—Unio margaritifer—as that of our bivalves, all of which live in the water, whereas of univalves some are “land snails” and some “water snails.” It would say of itself, “I am a species of the genus Unio (unio, a pearl), which belongs to the family Unionidæ, which belongs to the sub-order Isomya (i.e. having muscles of equal power to close the two valves of the shell), which belongs to the order Lamellibranchiata (i.e. having gills arranged in leaf-like fashion), which belongs to the sub-class Pelecypoda (i.e. having a foot somewhat of an axe-shape), which belongs to the class Acephala (headless), which is the second of the two chief classes into which Mollusca are divided.

“I differ from the Gasteropoda (whether they be terrestrial or aquatic) in that I and my near relations are exclusively aquatic and of a sedentary life, which makes the protection of two encompassing shells necessary. These shells are secreted by my mantle lobes, and are united by a ligament which tends to make the valves ‘gape’ for water and food and by two contracting muscles which close them in danger. I have a degenerate brain and no eyes. My mouth has neither jaw nor teeth, but possesses nervous lips covered with cilia, the vibration of which carries food-laden water to my mouth. My foot, when protruded, is seen as a large muscular appendage, and, by alternately expanding and contracting, it enables me to burrow or plough through mud or even sand, and so disturb the minute organisms on which I feed. I can thus travel fifteen feet a day, or about a mile in a year.

“I have no eyes, but distinguish well between light and shade by means of the surface of my body when exposed. I breathe, that is, get oxygen from the water, by means of gill-plates. As regards other internal organs, I differ not much from H. aspersa, but I am either male or female. Outside I am black and uncomely; but within I am pearly-white, and but for my power of forming pearls round an irritating grain of sand the civilization of England would have come to pass later than it did, for it was the report of my pearls which brought Cæsar to Britain.”