Snail vs. slug: The ultimate evolutionary trade-off of shell versus freedom

“ Molluscs live on land, in fresh water and in the ocean. No depths of the ocean abysses are too great for the octopi, no coast but has its many shells, hardly a pond or stream is without its mussels and pond-snails, and in all regions the land-snails and slugs abound.
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Body form and structure.—The molluscs are not to be mistaken for any other of the lower animals; they have a structure peculiarly their own. In them the body is not articulated or segmented as with the worms and arthropods, nor radiate as in the echinoderms, nor plant-like as with the sponges and polyps. ”

“ Remember the remark of Darwin, that the brain of the ant, although not much larger than a pin point, "is one of the most marvelous atoms of matter in the world, perhaps more so than the brain of man."
Closely allied to the crustacea is the sub-family of the mollusca, which includes the oyster, clams, and similar creatures; also the snails, cuttle-fish, slugs, nautilus, sea-squirts, etc., etc. Some are protected by a hard shell, while others have a gristly outer skin, serving as an armor, while others still are naked. ”

“ The snail and its relative, the slugs, belong to the Pulmonate (i.e. air-breathing) division of the Gasteropoda. The sea-slugs, in which, like the land slugs, the shell is absent or reduced, are relatives of the land snails. Some of those found on our own shores are handsome creatures, brilliantly coloured. Both groups fall under the division Euthyneura, while the majority of the marine univalves belong to the division Streptoneura (i.e. Gasteropods with twisted nerves) . ”

“ In many ways snails are very much like slugs, but they have a shell large enough to contain the entire body when the animal withdraws inside it. Several hundred different kinds of snails are found in North America, and many more in other parts of the world, varying in size from that of a small pinhead to that of a big walnut. Some are exceedingly numerous, others so rare and singular in their living-places that they are highly prized by conchologists. All snails lay eggs, usually in damp soil ”

“ The Argonaut, or paper nautilus, has been regarded as the analogue of the snail, which, like it, secretes an external shell for the protection of its soft body; and the octopus as that of the garden slug, which, having organs like those of the snail, as the octopus has organs like those of the shell-bearing argonaut, has no shell. The cuttles and squids may be compared to some of the sea-slugs, as Aplysia and Bullæa, and to some land-slugs, as Parmacella and Limax, which have an internal shell. ”

“ See these two snails: one hath a house, the other wants it; yet both are snails, and it is a question whether case is the better; that which hath a house hath more shelter, but that which wants it hath more freedom; the privilege of that cover is but a burden—you see if it hath but a stone to climb over with what stress it draws up that artificial load, and if the passage proves strait finds no entrance, whereas the empty snail makes no difference of way. Surely it is always an ease and sometimes a happiness to have nothing. ”

“ The mollusks, clams, and snails took an easier, down-hill road. They formed a shell, and it developed large enough to cover them. It hampered and almost destroyed locomotion and reduced nerve to a minimum. But nerves are nothing but a nuisance anyhow. And why should they move? Food was plenty down in the mud, and if danger threatened, they withdrew into the shell. They stayed down in the mud and let the world go its way. If grievously afflicted by a parasite they produced a pearl—to save themselves from further discomfort. ”

“ Then come a number of intermediate species, each with progressively smaller and thinner shells, till at length we reach the testacella, which has only a sort of limpet-shaped shield on his tail, so that he is generally recognised as being the first of the slugs rather than the last of the snails. You will not find a testacella unless you particularly look for him, for he seldom comes above ground, being a most bloodthirsty subterraneous carnivore who follows the burrows of earthworms as savagely as a ferret tracks those of rabbits ”

“ How does it come, though, that slugs and snails now live together in the self-same districts? Why, because they each live in their own way. Slugs belong by origin to very damp and marshy spots; but in the fierce competition of modern life they spread themselves over comparatively dry places, provided there is long grass to hide in, or stones under which to creep, or juicy herbs like lettuce, among whose leaves are nice moist nooks wherein to lurk during the heat of the day. ”

“ The truest and most snail-like snails are found in greatest abundance upon high chalk-downs, heathy limestone hills, and other comparatively dry places; while the truest and most slug-like slugs are found in greatest abundance among low water-logged meadows, or under the damp fallen leaves of moist copses. The intermediate kinds inhabit the intermediate places. Yet to the last even the most thorough-going snails retain a final trace of their original water-haunting life, in their universal habit of seeking out the coolest and moistest spots of their respective habitats. ”

“ The shells which serve as houses to land-snails, and which the animals close in winter by their opercula, or doors, are known to all. Many snails are not provided with shells, and they secure themselves by creeping under dead leaves, stones, or pieces of wood, or into the sod and the ground. ”

“ B is the body, soft and with a surface generally wrinkled or covered with small tubercles. F is the foot or muscular pad which forms the foot by the wavelike contractions of which it moves. H is the head, bearing the tentacles T1 and T2, of which the upper pair have the eyes, E. The mantle, M, makes the shell by secreting lime, etc. In it is the breathing orifice, BO, obvious in the slug, but in the snail nearly hidden by the shell. L in the snail is the spiral part, the liver, and it occupies a large part of the shell. ”

“ A snail may possess over twenty thousand tiny flinty teeth set on a ribbon so as to make a mowing-machine for the vegetable matter on which it feeds. With its aid also you might study the life-history of a mollusc from the egg onwards, and be able to determine by minute anatomical points whether two molluscs were of the same species or not—a matter in which the shape or appearance of the shell is not always a safe guide. ”

“ This can be done by holding your thumb nearly over the mouth of a watertap, while the shell is held in the left hand. Only adult shells should usually be taken, and those which are weather-worn or bleached should be neglected. In most the lip, or opening, of the shell will be hard if adult, and membranous if young; but experience alone will enable you to discriminate, especially where the young of one species is like the adult of another. ”

“ Five is the normal number of bands on the largest or body-whorl, although sometimes all run into one, and often one, some, or all are wanting. Where only one band is found—throughout the Helicidæ—it is usually that on the periphery or middle of the whorl, and a shell in which this band is wanting, while others are found, is a rarity. People are usually astonished, on seeing a good series of the colour [Pg 18] and variations of these two shells, how they vie with those of warmer regions. ”

“ Note also that most slugs have many variations in colour and markings.
Testacella haliotidea.—This genus of slugs forms a link between the naked slugs with rudimentary shells within, and the snails which live within their shells. The name Testacella, or little shell, was given by Cuvier in 1800, because this slug has a small shell at the end of the tail. Haliotidea means having a shell in the form of the marine shell Haliotis, the meaning of which again is the ear-shaped seashell, often called “Venus’ Ear.” It is subterranean in habit, and lives on worms. ”

“ Snails' eggs are nearly white and semi-transparent; the empty shells of young snails are very lovely when placed in a good microscope: the polariscope bringing out their exquisite prismatic tints.
The gardener one day brought in a testacella, or shelled slug. It fed upon earth-worms and was quite unlike the ordinary black or grey slug, of which we have, alas! countless thousands preying upon all the green things of the earth. This shelled slug was yellow, and seemed able to elongate its body very differently to any other species. ”

“ Now, some snails with such houses took to living in very damp and marshy places, which they were naturally apt to do—as indeed the land-snails in a body are merely pond-snails which have taken to crawling up the leaves of marsh-plants, and have thus gradually acclimatised themselves to a terrestrial existence. We can trace a perfectly regular series from the most aquatic to the most land-loving species, just as I have tried to trace a regular series from the shell-bearing snails to the shell-less slugs. ”

“ The worm may be indigenous, but the birds and flies invade the place, while the slugs, snails, and rodents come in through fence and wall. They attack one’s cherished plants, and, granting that those plants have life, why should they not be protected, as one’s poultry is from foxes, and their young from predatory cats? Naturalists grant plants to possess life, circulation, sleep, functions, and nerves; they grow, they blossom, they have young ”

“ Your garden looks exquisite. I hope you are quite well.”
“Tidy, sir, thank you kindly, tidy; and, yes, the garden do look well just now, if we could keep out the thieves.”
“Ah! yes, the birds, and slugs, and snails, and insects,” said the other with a soft, grave smile; “but we must not forget, gardener, that these poor things do not comprehend the difference between right and wrong. The fair fruits of the earth are growing in their path, and they do not understand why they may not freely eat.” ”

“ During the first year these soil animals are present but cause no problem. But after the first mild winter with no population setback, they become a plague. Slugs (and in California, snails) will be found everywhere, devastating seedlings. Earwigs and sow bugs, that previously only were seen eating only decaying mulch, begin to attack plants. It soon becomes impossible to get a stand of seedlings established. The situation can be rapidly cured by raking up all the mulch, carting it away from the garden, and composting it. ”

“ Upon the land the snails and slugs devoured the green foliage, while the vast army of insects filled every nook and cranny in the water, on the land, or in the air. Yes! even among these lower forms we found creatures enough to stock the world over and over again with abundant life, so that even if the octopus had remained the monarch of the sea, and the tiny ant the most intelligent ruler on the land, there would have been no barren space, no uninhabited tracts, except those burning deserts and frozen peaks where life can scarcely exist. ”

“ The usual modes of entrapping snails, slugs, and wood-lice, are laying down slices of raw potatoes or cabbage-leaves at night, and examining them before the dew is off the plants in the morning. As, however, this requires very early rising, a more convenient method is to lay a few flower pots upon their sides, where the snails have committed their ravages; and the snails, which can neither move nor feed unless the ground be wet with dew or rain, will generally be found to take refuge in the flower pots from the heat of the sun. ”

“ Slugs and snails are great enemies to every kind of garden plant, whether flower or vegetable; they wander in the night to feed, and return at daylight to their haunts. In order to catch them lay cabbage leaves about the ground, especially on the beds which they frequent. Every morning examine these leaves, and you will find a great many taking refuge beneath, and these may be killed by sprinkling them with a little lime or salt. ”

“ There must be something ghostly in the air of Christmas—something about the close, muggy atmosphere that draws up the ghosts, like the dampness of the summer rains brings out the frogs and snails. ”

“ As a rule, they wait till rain is over, probably objecting to the patter of the drops upon their delicate tentacles. Snails kept in captivity under a bell-glass are acutely sensitive of a damp atmosphere, and will bestir themselves after rain just as if they were in the open air. Certain Helices which are accustomed to live in moist places, will find their way to water, if removed from their usual haunts. ”

“ We were going round the garden in a procession, I with a big pair of scissors, and the Three with baskets, into one of which I put fresh flowers, and into the others flowers that were beginning to seed, dead flowers, and seed-pods. The garden was quivering in heat and light; rain in the morning had brought out all the snails and all the sweetness, and we were very happy, as we always are, I when I am knee-deep in flowers, and the babies when they can find new sorts of snails to add to their collections. ”

“ Our Cincinnati observer mentions three kinds of snails which move along at the rate of a mile in forty-four hours; they inhabit the most dense forests, and it is regarded as a sure indication of rain to observe them moving towards an exposed situation. Others indicate the weather not only by exuding fluids, but by the colour of the animal. After rain the snail has a very dark appearance, but it grows of a bright colour as the water is expended; whilst just before rain it is of yellowish white colour, also just before rain streaks appear from the point of the head to the jointure of the shell. ”

“ Snails never drink, but imbibe moisture during rain and exude it afterwards. They are seldom seen abroad except before rain, when they commence climbing trees and getting upon leaves. The tree snail is so sensitive to weather that it will commence to climb two days before the rain comes. If the downpour is to be prolonged, the snail seeks the under part of a leaf; but if a short or light rain is coming on, it stays on the outside. There is another species which is yellow before and bluish after it. ”

“ Though slugs are active throughout the year, and even during winter when the temperature is favourable, snails pass the winter, as well as hot, dry spells in summer, in a dormant state, often being found together in sheltered positions where the conditions are dry. ”

“ Mollusks in particular, like oysters and mussels, lead very monotonous and uneventful lives, only varied as a rule by the welcome change of being cut out of their shells and eaten alive; and their powers of living without food under adverse circumstances are really very remarkable. Freshwater snails and mussels, in cold weather, bury themselves in the mud of ponds or rivers; and land-snails hide themselves in the ground or under moss and leaves. ”

“ When winter draws near all the snails go into hiding, and they have a most curious way of closing the entrances to their shells by making little doors across them, composed partly of slime and partly of very small fragments of earth. ”

“ 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 [Pg 25] two exceptions in British molluscs) have not. ”

“ The American species are for the most part solitary, and in this respect differ greatly from their European congeners, which are social. Those, however, which have been introduced from Europe—and they are not few—retain their native habits. The appendages which perform the office of teeth for snails are peculiar in structure and various in form, and they do good execution on whatever eatables the animals may attack. The slugs are snails without external shells; are more nocturnal in their habits than the other snails ”

“ The food of the thrush is mostly of an animal character, as worms, slugs, and snails; and, by seasides, small molluscs, as whelks and periwinkles. On all such as are enclosed in shells he exercises his ingenuity in a remarkable way. We ourselves lived at one time in an old house standing in an old garden where were many ancient trees and out-buildings, in the old ivied roots and walls of which congregated great quantities of shell-snails. ”

“ Thrushes and mice are its great enemies, the former smashing it on some stone which may be found surrounded by the broken shells. The “mouth” or peristome is normally black, the shell larger and stouter than hortensis, in which the mouth is white. When a white-mouthed nemoralis or dark-mouthed hortensis (both rare) is found, the shape of the internal flinty dart at once distinguishes them. In some places both live together: in most one is found and not the other. ”

“ Nature's reasons were, however, all-sufficient; the little creatures, feeble and blind, needed a secure hiding place, screened from the changeful wind of night and from every roving enemy. The haymakers, moving to and fro amid the swathes, knew nothing of the hedgehogs' whereabouts; but when the dews of night lay thick on the strewn wild flowers, the parent “urchins,” leaving their helpless charges asleep within their nest, wondered greatly, while they hunted for snails and slugs in the ditch, at the quick change that had passed over the silent field. ”

“ Several individuals were together, and I imagined them to be proceeding in search of breeding-grounds, or on a migratory expedition. When disturbed by day, they fly with extended neck and dangling legs, and are easily shot, as their course is generally direct and their flight slow. When walking, it shoots its head forward at every step, as if about to thrust its bill into some substance; and, if you attempt to lay hold of it when disabled, it is apt to inflict a painful wound.
The food of this bird consists of snails, slugs, tadpoles, or young frogs and water-lizards. ”