Natural History, Mollusca/Gasteropoda
CLASS III. GASTEROPODA.
(Crawling Mollusks.)
If we examine the manner in which the common Garden Snail crawls, and especially if we look at it through a pane of glass as it glides up on the outside of the window, we shall see that the whole of the under-part of the body forms a wide fleshy disk, which is applied to the surface upon which the animal moves. Carefully watching this, we perceive that there are minute muscular movements constantly taking place over the whole disk, by means of which the animal advances with an uniform gliding progression. This disk, and its peculiar action, afford the most important character by which this Class of animals is distinguished, and hence they have been named Gasteropoda, which signifies belly-footed.
The upper surface of the body is covered with a fleshy cloak, the edges of which usually project in a greater or less degree, overlapping the foot-disk and other organs. This is called the mantle.
There is a distinct head, more or less conspicuous, according as it more or less projects from beneath the edge of the mantle; it is furnished with tentacles, varying in number from two to six, arranged in pairs on each side. These are probably delicate organs of touch, and perhaps they may be connected with other senses also. The majority of species are furnished with eyes, commonly placed either at the bases or at the tips of one of the pairs of tentacles. Every one is familiar with their appearance in the snails and slugs of our gardens, in which they are placed as minute shining black points at the tips of the upper tentacles. Many of the marine Gasteropoda, as the great Conchs (Strombus) of the tropical seas, have eyes well developed and of elaborate structure. Mr. Swainson says — "In the typical Strombi, these organs are so much developed that the iris is richly coloured, and the eyes of some of the larger species have been described to us as particularly beautiful."[1] According to the Rev. Lansdown Guilding, a naturalist who has enjoyed the advantage of familiarity with these fine Mollusca in their native seas, they have a distinct pupil and a double iris, equalling in beauty and correctness of outline those of birds and reptiles; and he discovers in the organ a vitreous and an aqueous humour, and the black pigment.[2] And Mr. J. E. Gray affirms, that "the eyes of the marine carnivorous Mollusca, Buccinum undatum, or Fusus despectus, and more especially some of the larger Strombi, are as fully developed as in the cuttle-fish, showing the cornea and the nearly orbicular crystalline lens almost perfectly formed, as may be seen by any person simply cutting the cornea across, and slightly pressing it, when the crystalline lens will protrude."[3]
Some species of this Class, few as compared with the great body, are naked, but the majority are protected by a shell, in some cases very thin, brittle, and glassy, in others somewhat horny, but more generally of a stony texture, and of great solidity and hardness. The chemical composition of these shells, however, varies very little; they consist of carbonate of lime deposited in cells of animal albumen. In the porcelain shells, of which the Cowries (Cypræa) afford familiar and beautiful examples, the lime is compact, with so small a portion of animal matter, that when immersed in acid the shell is completely dissolved, no sensible trace remaining. In the pearly shells, such as the genus Trochus, the calcareous matter is deposited in layers; and these, when submitted to acids, leave behind an insoluble membrane of albumen which retains the form of the shell.[4]
The shell is secreted by the mantle. In one family, that of the Chitons, it consists of several pieces, but in general it is simple, and takes the form of a hollow cone produced in various degrees. In the Limpets, which we see adhering so abundantly to our sea-side rocks, the cone is low and nearly symmetrical; but in the great majority of this class, the cone is greatly lengthened and twisted upon itself, so as to form a spire.
The mode in which the shell is formed has been well investigated by Mr. J. E. Gray, whose observations on the subject I shall here take occasion to cite:—
"The shell, which is peculiar to this division of the animal kingdom, may be seen covering the young animal in the egg before it has gained all its organs, as was observed by Swammerdam, and verified by the more extended observations of Pfeiffer, Turpin, and others. They are easily seen in the egg of the Limnæi, Physæ, Ancyli, and Bithiniæ, which have a transparent coat.
“The shells of the newly-hatched animals have been frequently considered as distinct species, and some very thin shells of land Mollusca, such as Vitrinœ, have been taken for the young of other well-known species, as Helix hortensis. These young shells are easily known by their always being of a pale horn colour; the whorls are generally rather irregular, and enlarge very rapidly; and the apex of the whorl, which was first formed, is generally large and blunt compared with the size of the shell. They are always destitute of colour, for the animal does not deposit the colouring matter until after it has been hatched; and it is therefore generally easy to distinguish, in the young shell, (and sometimes also in the adult,) that part of the top of the spire which formed the shell of the animal when in the egg.
“The shell is formed by the hardening of the animal matter, which is secreted by certain glands on the surface of the body, by means of chalky matter, which is also secreted by similar glands. It has been stated that the unhatched animal, very shortly after it is formed, begins to make its shell; and when it is hatched, deposits on the edge of the mouth of the little shell, which covered its body in the egg, a small quantity of the mucous secretion. This dries, and is then lined with some mucous matter, intermixed with calcareous particles; and when this hardens, it again places on its edge another very thin layer of the mucous secretion, and again lines it as before. The mucous secretion first deposited forms the outer coat of the shell, and is of use in protecting it from injury, while the mucous matter mixed with lime, which is placed within it, forms the substance of the shell itself. This deposition of mucous, and of mucous mixed with calcareous matter, goes on as the animal grows and feels the want of a larger shell for its protection. The shell is, in fact, moulded on the body of the animal itself, as the body grows; and, for this reason, any irregularity in the body is moulded in the shell.
"The animal has the faculty, also, of mending any break or injury that its shell may have received, if it is not of such a magnitude as to derange all the functions of the animal itself; and it mends them in the same manner as it forms its shell, — that is to say, by depositing first a coat of animal matter, and then lining it with mucous matter, mixed with chalk, to harden it. But as the animal is usually very desirous of getting the repairs done as quickly as possible, and is most probably damaged by the injury it has received, these repairs are generally much more roughly executed than the shell itself, and commonly destitute of regular colour.
"The particles, which vary the colour of the surface of the shell, are deposited while the shell is being increased in size, immediately under the outer mucous coat; and as these particles are also secreted by peculiar glands, the colour is always situated in a particular manner on each species, the glands being gradually enlarged, and gradually separated, but not changed in position by the growth of the shell. All the variations exhibited in the colouring of the different species, or in the different individuals of the same species, are produced by the permanent or temporary interruption of the action of these glands."[5] The part upon which the spire turns is called the pillar. It is sometimes solid, but sometimes it is hollow; in the latter case the perforation is named the umbilicus. When the spire is long, the shell is said to be turbinated, which is the common form; but in some genera, as Planorbis, the convolution takes place in the same plane, and the shell is flat, or even concave. Such shells are termed discoid. When the upper part of each turn or whorl envelopes or covers that which preceded it, the spire is said to be concealed.
In almost all species the convolution is towards the right side. There are a few, however, which turn to the left; these shells are termed reversed. The end of the latest whorl, where the animal protrudes, is termed the mouth or aperture. In order to close this, when the animal withdraws itself into its shell, the hinder part of the foot is usually furnished with a horny or shelly plate, called the operculum, which, when the animal contracts, is brought into such a situation, as more or less completely to close the mouth of the shell, when the animal is drawn into its cavity. It has hitherto been observed only in those Mollusca which have pectinate branchiae, and in two genera, Cyclostoma and Helicina, among the air-breathing land-shells. The form of the operculum is in general either that of a very low cone, made by successive layers, each one a little larger than its predecessor, or that of a flattened spire, and the texture is either horny or shelly.
The species of Gasteropoda are very numerous, and are arranged in five orders, viz.—Pulmonifera, Nudibranchiata, Tectibranchiata, Cyclobranchiata, and Pectinibranchiata.