Imatges de pÓgina
[ocr errors]

some protected by a multivalve tubular shell, the inhabitant protruding its organs at the summit, which is defended by an operculum consisting of more than a single piece-in others, also, the shell is multivalve, but the animal protrudes itself at the sides, and has no operculum, as in the common barnacle.2 Others, again, are protected by a shell consisting of two valves, open at one or two ends, and these seek farther protection either by burying themselves in the sand or perforating the rocks, or by suspending themselves by a byssus; others, again, which only open their shells at certain times, as the oyster, fix themselves to any convenient substance. To these succeed others, whose shell is transversely divided into many pieces,3 but yet, taken together, it forms a single valve protecting the back of a gastropod, or slug-like animal, which for farther protection when it is not moving, and to supply the place of a lower valve, fastens itself to a rock or other substance.

With the Patellidans begin the undivided univalve shells, which like the preceding animals protect their lower side by fixing themselves to the rocks; the sea-ears, which are stillTM more open, have recourse to a similar mode of protecting themselves; they preserve a communication with the atmosphere or water without elevating their shells, by means of a line of apertures under the thickest margin near the apex; these apertures begin when the animal is young near the spire and as it grows it stops up one and opens another, as its occasions require. I have a very large specimen, in which there are traces of eighteen apertures, and all but six are stopped up. If we turn our eyes from these to the Buccinidan or Whelk tribe, we are struck by an open Peruvian shell, which at first sight scems like a lirapet, but upon inquiry we find that it is defended by an operculum, the plan of protection being here changed, and, instead of an under-valve, or a rocky munition. it is closed by a broad plate, which some peculiarity in its structure and organization doubtless required; from this by Purpura and Monoceros to the true Buccinum, the mouth narrows and the operculum with it.


If we examine the common periwinkle, we find the mouth of its shell closed by a horny organ called the patch, which is attached to the foot or rather neck, by its convex or lower surface, sitting on a sub-triangular flat space spirally convoluted; this is the operculum, and if examined on either side will be found to be also spirally convoluted, proving that it is formed.

3 Chiton.

1 Balanus. Tubicinella.

4 Haliotis.

2 Pentelasmis.
5 Concholepas.

by the part on which it sits. When the animal expands its foot for creeping, the operculum is retracted within the shell, so as to be quite out of the way. If we examine the opercula of other shells, we shall find that the majority of them have the same spiral configuration traced both on the upper and lower surface. In most that I have seen the intervals of the whirls increase in width, as the spires of the shells do from the base to the mouth. In the top-shell' the whirls are perfectly regular and nearly equidistant. They vary much in thickness; I have one three-fourths of an inch thick, while those of the top-shell and periwinkle are very thin. In some of the thick ones, on the under side the convolutions are very convex, and sometimes elevated into concentrical ridges. Some underneath have a forest of obtuse elevations, and many are rough with minute tubercles. As to substance, some are horny, while others resemble the shell; others are horny externally and shelly internally. If these formations on the under side, as in the common periwinkle, represent the shape of the part of the neck to which they are attached, as they most probably do, it must act the part of a mould, upon which the operculum is formed from its mucus, and increased as the aperture enlarges.

Lamarck is of opinion that the shell of univalves is formed in a similar way upon the neck of the animal, which in the Murices or rock-shells, and other tribes distinguished by spines or tubercles, has certain fleshy processes which produce those spines, &c. and is withdrawn when they have acquired consistence enough not to bend when thus left to themselves. Other conchologists, particularly one of the most eminent of our times, Poli, think that the shells of univalves are organized bodies, and produce their spines as vegetables do their prickles, he says also that their shells contain cellular membranes al most like a Rete mucosum.

In the progress of a shell's growth, as new spines are formed old ones drop off: how this is effected seems not to be accounted for by either hypothesis-it is analogous, however, in a great degree, to what was mentioned above with regard to the holes in the shell of the sea-ear, only that with them an old hole is stopped up, when a new one is formed. All that can be said on the subject is that the animal, instructed by Providence, as new processes are formed and a new whirl of its shell completed, is enabled to throw off by a solvent, or some other means unascertained, those that are no longer wanted.

It is observable that the terrestrial univalves,' of this Order,

1 Trochus.

2 Helix, &c.

are never armed with spines, tubercles, or other elevations, but exhibit generally a levigated shell. As they move about usually amongst bushes, under moss, or in grass, the object of the Creator in this structure was probably that their motions might not be impeded by any roughness of their shell.

Mr. E. W. Brayley, in a very ingenious memoir, in the Zoological Journal, has contended, with considerable strength of argument, that the moveable black points, in the upper tentacles of snails, though he allows they may be their analogues, are not real eyes; but the Rev. L. Guilding, in a subsequent part of the same Journal states, that the large strombs of the Caribbean sea have eyes furnished with iris and pupil, similar to those of birds and reptiles-that they have also a vitreous and aqueous humour, and a black pigment, which certainly prove them to be real eyes-their organ of hearing, he thought, was likewise distinct. The cowrics also are said to have eyes exhibiting both iris and pupil, as have some volutes.1

Giving these facts their due weight, I think we may conclude that the, so called, eyes of snails, are real though imperfect visual organs. It appears to be the plan of the Creator,

to ascend

From small beginnings to a glorious end.

An organ is, as it were, sketched out, in the lowest animal, as for instance, a nervous system, which keeps developing and improving till it is brought to its acme in the highest first we find in the polypes no nervous centre, but molecules every where dispersed; then the next form is a nervous collar round the esophagus; next dispersed ganglions; then a ganglionic chord; and so on till we arrive at a regular brain and spinal marrow incased in a vertebral column. We may with reason therefore conclude, that the organ of vision, when first planted, would be a mere rudiment, though sufficient for the animal's purposes, and possessing few of the characters it exhibits when arrived at its most perfect form; these it keeps acquiring, as it becomes more developed, or to avoid misconception from nibbling critics, the Creator keeps giving it more and more perfect sight till he brings it forth, in all its glory, in the highest animals.

The most common in this country of these herbivorous Trachelipods, is the garden-snail, but the species whose history has been most copiously related, is that called in France the

1 Voluta Ethiopica, PLATE VI. FIG. 1. a.

2 Helix hortensis.

[ocr errors]

Escargot, which, though stated to have been originally imported into this country, now abounds in some parts of Surry and other southern counties. I shall begin by giving some account of their economical and then of their physical history.

On the continent, especially in France, this large srail, which is more than double the size of our garden one, is used as an article of food, and though said not to be easy of digestion, is very palatable. They are thought to be in best season in the winter, when they are hybernating, and covered with their temporary calcareous operculum, which falls off in the spring. The Romans appear to have fattened these snails, in places appropriated for that purpose. Pliny mentions several sorts that were kept separate, and amongst others white ones that were found in the neighbourhood of Rieti. The Illyrian snails he describes as the largest; the African as most prolific; others from Soletum, in the Neapolitan territory, as the noblest and best: he speaks of some as attaining to so enormous a size, that their shells would contain eighty pieces of money of the common currency. Bruguières, to whom conchology is under very great obligations, is of opinion that, by cultivation, the several species of snails might be brought to a much greater size, and furnish an abundant, wholesome, and even delicate aliment. There is no reason why the species of this genus, which feed on vegetable substances, should not be as palatable as the oyster or periwinkle.

1 H. Pomatia.

Snails, in general, are hermaphrodites, or unite both sexes in the same individual: this is the case with the great majori ty of Molluscans; the object of Providence, in this kind of organization, is evidently the greater multiplication of the species, but though hermaphrodites, in each individual possessing the organs of both sexes, they are not so as to sexual union; reproduction can only take place when different individuals impregnate each other; this union takes place at the beginning of the spring, sooner or later, according to the heat of the season. Their courtship is singular, and realizes the Pagan fable of Cupid's arrows, for, previous to their union, each snail throws a winged dart or arrow at its partner. About twenty days after coupling the snails lay, at different times, a great number of white eggs, varying at each laying from twenty-five to eighty, as large as little peas, enveloped in a membranous shell, which cracks when dried. They lay these eggs in shady and moist places, in hollows which they excavate with their foot, and afterwards cover with the same organ.

2 Quadrans.

These eggs hatch, sooner or later, according to the temperature, producing little snails exactly resembling their parent, but so delicate that a sun-stroke destroys them, and animals feed upon them; so that few, comparatively speaking, reach the end of the first year, when they are sufficiently defended by the hardness of their shell. The animal, at its first exclusion, lives solely on the pellicle of the egg from which it was produced. Providence, which in oviparous and other animals, has provided for the first nutriment of the young in different ways, appropriating the milk of the mother to the young of quadrupeds; the yolk of the egg to those of birds, tortoises, and lizards; and the white of the egg to frogs and toads, has made this pellicle or coat the best nutriment of the young snail. In fact, this pellicle, consisting of carbonate of lime, united to animal substance, is necessary to produce the calcareous secre tion of the mantle, and to consolidate the shell, as yet too soft for exposure. When this envelope is eaten, the little snail finds its nutriment, more or less, in the vegetable soil around it, and from which it continues to derive materials for the growth and consolidation of the shell. It remains thus concealed for more than a month, when it first issues forth into the world, and without respect of persons, attacks the vegetable productions around, returning often to an earthly aliment, probably still necessary, for the due growth and hardening of its portable house. These snails cease feeding when the first chills of autumn are felt, and associating, in considerable numbers, on hillocks, the banks of ditches, or in thickets and hedges, set about their preparations for their winter retreat. They first expel the contents of their intestines, and then concealing themselves under moss, grass, or dead leaves, each forms, by means of its foot, and the viscid mucus which it secretes, a cavity large enough to contain its shell. The mode in which it effects this is remarkable; collecting a considerable quantity of the mucus on the sole of its foot, a portion of earth and dead leaves adheres to it, which it shakes off on one side; a second portion is again thus selected and deposited, and so on till it has reared around itself a kind of wall of sufficient height to form a cavity that will contain its shell; by turning itself round it presses against the sides and renders them smooth and firm. The dome, or covering, is formed in the same way: earth is collected on the foot, which then is turned upwards, and throws it off by exuding fresh mucus; and this is repeated till a perfect roof is formed. Having now completed its winter house, it draws in its foot, covering it with the mantle, and opens its spiracle to draw in the air. On closing this, it forms.

« AnteriorContinua »