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Natural History: Mollusca. Philip Henry Gosse
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isbn 4064066065522
Автор произведения Philip Henry Gosse
Жанр Документальная литература
Издательство Bookwire
In the Tunicata, examples of which may be found on our rocky beaches, closely adhering to the under surface of stones at low water, and looking like shapeless masses of a substance something between gristle and jelly—the breathing organ is developed to a very great extent. It occupies a capacious chamber in the interior of the animal, the two sides of which are studded on their inner surfaces with little oval cells, arranged in a regular pattern of rows. Each of these cells is formed by an oval ring of cilia, which, when in full play, present a most beautiful and interesting spectacle. The accompanying figure, taken from the life, is a magnified representation of a tiny creature, not larger than a pin's head, but as transparent as the purest crystal. The oblong rings conspicuously seen are the ciliary cells of the breathing organ; but no figure can convey an adequate impression of the beauty of the sight, when the observer gazes upon forty or more of these ovals, all set round their interior with what look like the cogs on a watch-wheel, dark and distinct, running round and round with an even, moderately rapid, ceaseless motion.
One large tribe of the Gasteropoda comprises animals, however, which breathe air, and are terrestrial in their habits. Of these the Slugs and Snails of our fields and gardens afford familiar examples. The delicately-formed, and often brilliantly-painted shells, which throng the damp woods in tropical countries, likewise belong to this group, and furnish the most highly prized treasures of our conchological cabinets. In these the air is
PEROPHORA.
inhaled into an ample chamber, the interior of which is lined with cilia. After parting with its oxygen, it is again expelled; the common orifice for both processes being situated on the side of the body. Any one may have ocular evidence of the existence of this organ, by watching our common Garden Snail. If you look at its right side, just behind the tentacle, or horn, that carries a black eye at its point, you will see a large hole suddenly open, where before there was no trace of it. After remaining open for a few moments, the margin will leisurely contract again, until it is perfectly closed, and as invisible as before. This is the breathing orifice; and during the interval that you saw it open, the aerial contents of the chamber were expelled, and a copious draught of fresh air was inspired. The process is repeated with tolerable regularity about once every fifteen seconds.
The blood in the MOLLUSCA is thin, transparent, and colourless; or at most presents only a pale bluish-white hue. It is, however, contained in a system of distinct vessels, through which it circulates, having for the source of its motion a well-developed, complex, pulsating heart.
Besides the system of vessels which carry the blood, there is another system, most conspicuous in the aquatic tribes, which has been called the system of aqueducts. They communicate with the element in which the animal lives and moves, and are filled with it at will, as the galleries and canals of a sponge are filled with the liquid in which it is immersed. The chief use of these water-canals appears to be the distension and expansion of the foot, to render it better fitted for locomotion, yet so as not to interfere with the privilege, essential to most of these animals, of withdrawing the whole of the body within a shell. Some of the marine Mollusca, when in a state of activity, protrude a soft foot, far exceeding in dimensions the whole bulk of the shell; yet let the creature be disturbed, and the whole is suddenly withdrawn into the cavity, so completely that not a trace of it is visible. "When shrunk within its shell," observes Dr. Johnston, "you might well deem any animal that could hide itself there, all too small and weak to carry about a burden larger and heavier than itself,
THE GIANT STROMBUS.
and that safety might be here advantageously exchanged for relief from so much heaviness of armour, and from such an impediment to every journey. There is in my small cabinet a fine specimen of Cassis tuberosa, which measures fully ten inches in length, and upwards of eight in breadth, another of Strombus gigas is nearly one foot in length. The weight of the former is four pounds two ounces; that of the latter, four pounds nine ounces; yet the snail creeps under this load at apparent ease. Nor are you much surprised when you see it actually in motion, for the seeming disproportion between the contained animal and containing shell has disappeared. On issuing from its shell, like an Eastern Genii freed from his exorcism, the animal has grown visibly—has assumed a portlier size and more pedestrious figure. The body has suddenly become tumid and elastic, the skin and exterior organs stretched and displayed, the foot has grown in length and in breadth, and, with additional firmness, it has acquired at the same time the capability of being directed, bent, and modified in shape, to a considerable degree, as the surface of the road traversed may require. Thus it is with nearly all the cephalous mollusca; and by a similar disposition of aqueducts, the foot of the Bivalves is equally adapted to every act subservient to their locomotion and more especially to the act of burrowing; for had the foot not been so framed as to permit of an enlargement superior to the size of the shell, it seems obvious that the furrow could not have been made large enough to contain the latter. The same, too, with many Gasteropods which burrow in the sand when in search of prey. The Buccina and most carnivorous mollusca have this ability, dependent on the system of aqueducts we have been describing; and you must observe, that from the manner in which the shell is attached to the body by the large retractor muscle, it so happens that this is drawn into the furrow always with the notch in the aperture uppermost, so that, when completely buried, the animal is still enabled to communicate with the water by its respiratory siphon."[4]
Beyond the rudimentary strip of cartilage that in some of the Cephalopoda represents the vanishing spine of the Vertebrata, the Mollusca have no internal skeleton. But in the great majority of cases, the soft parts are protected and supported by what we may call an external skeleton, of the substance familiar to us as shell. Lime is the essential element of this substance, as it is also of bone: but shell is a carbonate of lime, while the earthy part of bone is a phosphate.
When we consider the beauty and variety that are presented by shells, the important part they play in the economy and habits of the animals, and the use that is made of them in systematic arrangement, it becomes a question of high interest to inquire in what manner they are formed.
"The shells themselves are absolutely deprived of vitality, permeated by no vessels, and as incapable of expansion by any internal power as the rocks to which they are not uncommonly attached; so that the young naturalist is necessarily at a loss to conceive either the mode of their formation, or the origin of all the gaudy tints and external decorations that render them the ornaments of our cabinets.
"The simple apparatus by means of which shells are constructed, is the external membranous layer that invests the body of the mollusk—the mantle, as it has been termed; and, whatever the form of the shell, it owes its origin entirely to this delicate organ. … .
"It is the circumference, or thickened margin of the mantle alone which provides for the increase of the shell in superficial extent. On examining this part, it is found to be of a glandular character, and moreover not unfrequently provided with a delicate and highly sensitive fringe of minute tentacula. Considered more attentively, it is seen to contain in its substance patches of different colours, corresponding both in tint and relative position with those that decorate the exterior of the shell.
"When the animal is engaged in increasing the dimensions of its abode, the margin of the mantle is protruded, and firmly adherent all round to the circumference of the valve with which it corresponds. Thus circumstanced, it secretes calcareous matter, and deposits it in a soft state upon the extreme edge of the shell, where the secretion hardens and becomes converted into a layer of solid testaceous substance. At intervals this process is repeated, and every newly-formed layer enlarges the diameter of the valve. The concentric strata thus deposited remain