Natural History, Mollusca/Dibranchiata

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Natural History, Mollusca by Philip Henry Gosse
Dibranchiata

ORDER DIBRANCHIATA.

(Two-gilled Cuttles.)


The creatures composing this order approach nearest to the Vertebrate animals of all the Mollusca. They have a distinct brain included in a box of cartilage—the vanishing remains of a bony skull; they have large highly-coloured and complex eyes, protected in some species by eyelids; and ears of simple structure, hollowed in the cartilage of the rudimentary skull. They are remarkable for having three separate and well organized hearts, one for the circulation of the arterial blood through the body, the others for the projection of the venous blood through the two gills.

Any person who has had an opportunity of examining one of these animals in a living state, must have been struck with a very curious phenomenon. Over the whole surface of the body there are coloured spots which are perpetually changing their position and figure, running into each other and separating, playing hither and thither, contracting and dilating, appearing and disappearing, with great velocity and in the most singular manner. On close examination, it appears evident that these changes are owing to a fluid which moves irregularly within the substance of the skin. Even after death the spots continue to play for a considerable time, and that on small portions of the skin cut away from the rest.

The cause of this curious appearance is not yet thoroughly understood. Milne-Edwards, whose opinion is entitled to the highest respect, gives the following explanation of it:—"The skin of these animals is furnished with a number of differently-coloured spots, which alternately appear and disappear, and if a portion is put under a microscope, it may be perceived that these changes depend on the contraction of small vesicles filled with a coloured liquid, which reach from the surface of the skin to a considerable depth. When one of these spots appears, the liquid, corresponding here to the pigment in the other case, is propelled towards the superficial part of the vesicle, and there displays itself; whilst during its disappearance it is forced into the deeper parts by the contraction of this superficial point itself, which then becomes almost invisible."[1]


Family Sepiadæ.

(Cuttles and Squids.)

The lingering rudiment of a vertebrate skeleton in these animals has been already noticed; their body encloses, however, a solid support of quite another nature, which represents the true shell so characteristic of Mollusca generally. Within the substance of the mantle, if we slit it up along the line of the back, we find an oblong cavity, within which lies loose, and unconnected with it a large plate, horny in some species and shelly in others. The pen of the common Squid (Loligo vulgaris) is of the former texture; the substance called cuttle-bone, so often found on sandy beaches, is of the latter; but both are strictly analogous to the shell of the slug, which is also enclosed within the mantle; and both are formed in the same manner, namely, by a deposition of horny or shelly matter in a fluid state from the sides of the containing cavity.

The animals of this Family have ten arms, two of which, greatly longer than the rest, are very slender except near their tips, which are dilated; these extremities alone are furnished with suckers. The other eight arms are short, thick, and furnished throughout their whole length with suckers, forming a double row along their under surface.

The body is generally lengthened, more or less flattened, with the skin dilated on each side so as to form a pair of wings or fins. It is probably by means of the impetus afforded to the body by these expansions, that some species of the Family are enabled to throw themselves out of the water, and to shoot along through the air to a considerable distance with a motion that resembles flight. These are commonly called Flying Squid.

Mr. F. D. Bennett describes a portion of the Northern Pacific as peculiarly animated by the presence of various oceanic creatures. The Albacore, the Sword-fish, the Barracuda, the Bonita, the Flying-fish, are mentioned, and among them the Squid, whose movements closely resembled those of the last-named volatile fish. "During a calm, in lat. 30° N., the Flying-squid appeared in larger flights than we had ever before witnessed; persecuted probably by the Albacore (which select this tranquil time to descend deep in the water, and to rove far from the ship in quest of food) they rose from the sea in large flocks, leaping over its smooth surface, much in the same manner, and to the same height and distance, as the Flying-fish. Many of them were captured by birds during their leaps, and one individual, in making a desperate effort to escape some aquatic pursuer, sprang to a considerable height above the bulwarks of the ship, and fell with violence upon the deck."[2]


Genus Sepia.

(The Cuttle.)


CUTTLE-BONE.
In this genus, which contains our commonest species of the Order, the body is oblong and flattened, with the side-fins extending along its whole length. The mantle is free at its front margin; the suckers are supported by horny hoops with entire edges. The internal support is shelly, and is composed of a succession of extremely delicate plates, sustained by slender columns, regularly arranged, the spaces between the plates being filled with air.

Many of my readers are doubtless familiar with the object here represented, so frequently cast up by the waves upon our smooth sandy beaches: it is the shell of the common Cuttle-fish (Sepia officinalis). Its use is not only to give firmness to the soft and jelly-like body of the animal, but to aid it in swimming by its buoyancy; for though the material of which it is composed is stone, from the delicacy of its texture and the peculiar arrangement of the plates, the large proportion of air enclosed within it renders the whole lighter than water.


CUTTLE.
The Cuttle is about a foot in length, of an oblong form. Its colour is a dull, dirty white, mottled and spotted with those changing veins of fluid, already described, of a reddish brown hue. The texture of the body is soft and flabby, but, notwithstanding its unpleasing appearance, it forms a wholesome and agreeable dish wherever prejudice does not preclude its use. When well cooked the flesh is tender and digestible, bearing considerable resemblance to tripe.

I have already alluded to the inky fluid secreted in an internal reservoir within the body of the Cuttle. It is poured forth in copious quantity from a funnel-like tube beneath the mantle, and is intended as a means of concealment, and of annoyance to its pursuers. "A gallant officer who was inconsiderately collecting shells in a pair of immaculate white trowsers, came suddenly upon one of the naked Cephalopods snugly harboured in a recess in the rock. They looked at each other, and the Cuttle, who had his eyes about him, and knew well how to use them, upon seeing the enemy advance, took good aim, and shot so true that he covered the snowy inexpressibles with the contents of his ink-bag, and rendered them unpresentable either in drawing-room or dining-room."[3]

Entangled among the sea-weeds washed up on the sea-beach in the latter part of summer, we occasionally see what at first sight we are ready to take for a bunch of purple grapes. The fisherman indeed calls them sea-grapes, so close is the likeness in colour, size, and aggregation. But if we take the cluster into our hand and examine it, we shall see that their texture is leathery, or somewhat like India-rubber, that the extremity of each berry runs out to a point, and that its base springs from a fleshy cord which clings and entwines irregularly around the marine plants. These berries are the eggs of the Cuttle-fish, and if we were to open the tough skin of one, we should find either the white yolk and clear glaire, or else the infant animal, perhaps fully formed and ready to take advantage of this premature opening of his prison, by darting out, with all his organs perfected and all his wits about him.

The parrot-like beak presents a strong exception to the general softness of this animal; it is so hard, stout, and stony, and moved by such powerful muscles, that the strong shells of bivalves and univalves are not able to resist its force: even the hard and stony limpet is dragged from its attachment, and crushed to pieces in these powerful mandibles.


  1. Edinb. New Phil. Journ. XVII. 319.
  2. Whaling Voyage round the Globe.
  3. Penny Cyclop. art. Sepiadæ.