Natural History, Mollusca/Tectibranchiata

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Natural History, Mollusca by Philip Henry Gosse
Tectibranchiata

ORDER TECTIBRANCHIATA.

(Covered-gilled Mollusks.)


In general form and appearance, many of the animals contained in this Order resemble those of the preceding; but they differ from them in having the gills attached along the right side, or upon the back. These organs assume the form of a single plume, or of leaflets more or less divided, but not symmetrical; they are always more or less covered by the mantle, whence the name applied by Cuvier to the Order. The mantle encloses within its substance, in almost all cases, a small shell to protect the vital parts. In some of the genera, the shell is developed to such a degree as to cover the animal; as in the fresh-water limpet (Ancylus), found in some of our streams. For the most part, however, the species are marine; they are widely scattered, but appear to be most numerous in the Indian and Mediterranean Seas. The sexual functions are united in each individual, in which particular, this Order agrees with those which I have already considered; but in the form of the breathing organs, it manifests a closer affinity with the following Order.

The Covered-gilled Mollusca may be grouped in five families, three of which are represented by British species.


Family Pleurobranchidæ.

The members of this family a cursory observer would at once associate with the Nudibranchs; but an examination of their structure reveals particulars in which we see an evident departure from that model. The mantle is large, projecting on all sides over the foot, which is also ample; thus a deep furrow runs all round between these parts. Within this furrow, on the right side, lies the branchial plume, resembling a series of pyramids, divided into triangular leaflets. The plume is attached to the body at its base, but its tip, which points backwards, is free. The organs of reproduction and of excretion are also situated on the same side, the former before, the latter behind the gill-plume. The head is distinct, concealed by a broad veil, which is furnished with four tubular tentacles. The mouth consists of a simple muscular proboscis, or fleshy tube, which is capable of considerable elongation and contraction; it is entirely destitute of teeth or any cutting instrument, but is, nevertheless, fully able to seize and force into the stomach such materials as are used for food. The stomach is greatly developed, and in the principal genus is divided into four cavities, of which the second is muscular, and armed with shelly teeth; the internal surface of the third is increased by longitudinal plates.

In the typical genus, there is a small thin shell enclosed within the mantle, and situated in the middle of the back, so as to cover and protect the viscera. In the curious genus Umbrella, from the Indian ocean and the Mediterranean, the shell is comparatively large, external, and shaped somewhat like that of a Limpet; but in Pleurobranchæa, there is no trace of a shell either internally or externally.

Genus Pleurobranchus.

A not unapt comparison has been made of these Mollusks to a tortoise, the mantle representing the back shield, and the foot the breastplate. They are more or less oval in outline, flattened, with the back convex; the mantle is fleshy, and projects considerably on all sides; the head is placed beneath its front margin, and carries two superior tentacles with a pair of minute eyes, sunk beneath the skin at the outer part of their bases.
GILL-PLUME OF PLEUROBRANCHUS.
The upper part of the head is developed into a broad triangular veil, the lateral edges of which carry another pair of tentacles. These organs, as well as the superior pair, are imperfectly tubular, and are formed by the bending together of their sides, which meet and overlap, but do not unite. The mouth is a short, fleshy proboscis, provided with horny jaws and a long ribbon-like tongue, armed with teeth, as in the Nudibranchs. The branchial plume projects from between the mantle and foot; in crawling it is transverse, and appears, when viewed from above, to be composed of many triangular laminæ, overlapping and pointing backwards. Each lamina shows transverse wrinkles. Viewed laterally, it is seen to consist of a central stem, with about eighteen pinnæ on each side, each pinna being again pinnated on each side. The stem, pinnæ, and pinnulæ, are all dilated inwardly, so that the stem, which is narrow and slender in one aspect, is wide in another; and the pinnæ are the triangular laminæ, whose wrinkles are in fact the pinnulæ. The organ is connected with the bottom of the lateral groove, for about two-thirds of its length, by a membrane. The plume can scarcely be recognised in its two aspects, even though examined again and again in quick succession. It appears very sensitive, and changes much in appearance by its various degrees of contraction and expansion.

The mantle contains in the centre of its substance an oval shallow cavity, within which lies, quite free and unattached, a shield-like shell of the same form, so delicate in its texture as to be almost membranous, with a very slight indication of a spire at one extremity. The position of the shell is indicated externally by a dark cloudy spot in the middle of the back; and on an incision being made in this part, the shell falls out.

The warmer seas produce the largest and most beautiful species of this genus, some of which are marked with bright colours. They are found swimming in the open ocean, and crawling on the rocks or weeds of the coast, and specimens have been dredged, from various depths to thirty fathoms, on stony bottoms and beds of sea-weed. We have two native species, neither of which can be considered common. The rarer of these, Pleurobranchus plumula, is found on our south-western coast, where it was first discovered by that eminent zoologist, Colonel Montagu. I had recently the good fortune to find two specimens in a rocky cove near Torquay, both of which lived in captivity for some weeks.

The species is usually described as yellowish white; but my specimens were both of a rich golden yellow, with the central cloud brown, tinged in front with red, the underside of the foot of a light flesh colour. The length of the larger was an inch and two-thirds when crawling, the breadth three quarters of an inch.

Natural History - Mollusca - Pleurobranchus plumula.png

PLEUROBRANCHUS PLUMULA.
(1. Viewed from above; 2. from beneath.)

In captivity they were sluggish, fond of hiding among the fronds and leaves of sea-weeds, but at times gliding freely like a Doris. They swam on the surface by the foot reversed, and then left behind a great wake of clear viscid jelly. They were beautiful animals. After keeping them in health about a fortnight, I put one into fresh water to kill it, for preservation. This, however, was not so readily fatal to it as I had supposed, for at the end of half an hour I found, by its contraction when touched, that it was still alive. Probably the mode in which it had contracted, on being put in—the foot being narrowed, and the edges of the mantle being incurved on all sides around the foot—may have in some measure prevented the access of the water to the vital parts. At the end of that time I replaced it in sea-water, where it soon partially recovered its activity, relaxed its mantle, and contracted it dorsally so as to expose the groove between it and the foot, greatly protruding the tentacles and veil, and the branchial plume; thus I was enabled to get a much better sight of these parts. As soon as it was replaced in the sea-water, a quantity of white mucus was discharged from the whole surface, most copiously from the foot, which as it lay on its back was uppermost. This mucus gradually, by the contractions of the animal, was accumulated in a knob at the posterior end of the foot, and then thrown off. The reticulate structure of the mantle integument was much more distinct than in health; it was seen to form a delicate lacework of yellow fibres all over the surface, covering and enclosing a pellucid parenchyma.

The animal evidently had been injured by its bath of fresh water; for it lay on its back, expanding and contracting its various parts, without the power of turning over to crawl, or even of adhering by the foot when turned, but rolling helplessly back. The form and appearance, too, were very different from those of health; the groove being widely exposed by the contraction of the mantle, in which state the animal resembled some published representations of it more closely than when in health.

The student of nature, one who really delights to contemplate the wonderful works of God, as they appear in the conditions and under the circumstances in which He has placed them, and not merely their dried and shrivelled remains, technically labelled and arranged in the drawers of a cabinet, can scarcely have a greater treat than a ramble on a summer's day along the margin of the sea, on some one of our rocky shores.

"'Tis pleasant to wander along on the sand,
   Beneath the high cliff that is hollow'd in caves,
When the fisher has put off his boat from the land,
   And the prawn-catcher wades thro' the short rippling waves;

While fast run before is the sandling and plover.
   Intent on the crabs and the sand-eels to feed;
Or on a smooth rock which the tide will soon cover,
   To find us a seat that is tap'stried with weed."

But still more pleasant is it to peer into those wells of pure water which are hollowed out in the living rock, fringed with waving sea-plants, and stocked with animals of various kinds, all pursuing their natural avocations, and disporting themselves in a thousand ways, under the umbrageous shade of what to them is a marine forest. As we gaze down into these clear, quiet depths, we almost unconsciously repeat the words of one of our noblest poets, who has selected such a scene for the embellishment of the wildest of his romances:—

      "And here were coral-bowers.
         And grots of madrepores,
And banks of sponge, as soft and fair to eye
          As e'er was mossy bed
    Whereon the wood-nymphs lie
With languid limbs in summer's sultry hours.
        Here too were living flowers.
     Which, like a bud compacted.
     Their purple ctips contracted.
   And now, in open blossom spread,
Stretch'd like green anthers many a seeking head.

    And arborets of jointed stone were there,
 And plants of fibres fine as silkworm's thread;
        Yea, beautiful as mermaid's golden hair
           Upon the waves dispread.
        Others that, like the broad banana growing,
Rais'd their long wrinkled leaves of purple hue.
        Like streamers wide outflowing."[1]

A hundred times might you fancy you saw the type, the very original of this description, tracing, line by line, and image by image, the details of the picture; and acknowledging, as you proceed, the minute truthfulness with which it has been drawn. For such is the loveliness of nature in these secluded reservoirs, that the accomplished poet, when depicting the gorgeous scenes of eastern mythology; scenes the wildest and most extravagant that imagination could paint; drew not upon the resources of his prolific fancy for imagery here, but was well content to jot down the simple lineaments of nature, as he saw her in plain homely England.

It is a beautiful and fascinating sight for those who have never seen it before, to see the little shrubberies of pink coralline,—the "arborets of jointed stone,"—that fringe these pretty pools. It is a charming sight to see the crimson bananalike leaves of the Delesseria waving in their darkest corners; and the purple fibrous tufts of Polysiphoniœ and Ceramia, "fine as silkworm's thread." But there are many others which give variety and impart beauty to these tide-pools. The broad leaves of the Ulva, finer than the finest cambric, and of the brightest emerald-green, adorn the hollows at the highest level; while at the lowest wave tiny forests of the feathery Ptilota and Dasya, and large leaves cut into fringes and furbelows, of rosy Rhodymeniæ. All these are lovely to behold; but I think I admire as much as any of them, one of the commonest of our marine plants, Chondrus crispus. It occurs in the greatest profusion on our coast, in every pool between tide-marks,

Natural History - Mollusca - Chondrus.png

CHONDRUS.

and every-where,—except in those of the highest level, where constant exposure to light dwarfs the plant, and turns it of a dull umber-brown tint,—it is elegant in form, and brilliant in colour. The expanding fan-shaped fronds, cut into segments, cut, and cut again, make fine bushy tufts in a deep pool, and every segment of every frond reflects a flush of the most lustrous azure, like that of a tempered sword-blade.

I have said that animals of various kinds inhabit these rock-pools. They are cavities of irregular shapes and diverse dimensions in the surface of the rock, covered by the sea at every incoming tide, and left full when it recedes. The water, therefore, presently becomes as clear as crystal, and the surface being too small to be ruffled by ordinary breezes, the eye can easily penetrate even to the bottom, and mark all that is going on within. There are little fishes, with bright eyes and silvery sides, peeping from under the shelter of the broad leaves, or darting out with vibrating fins from beneath one projection of the rock to another. Elegantly painted prawns are swimming leisurely to and fro, and hundreds of other smaller Crustacea are playing about. Sea anemones of different species stud the rocky sides, and attract the eye with their brilliant colours—crimson, purple, scarlet, green, and white—resembling gorgeous flowers, or ripe and mellow fruits, according as they are expanded or contracted. The shelled Gasteropods are not wanting; the little Cowry, the Purpura, the various species of Trochus, to say nothing of limpets and periwinkles. And here we may often see the lovely Nudibranchs and Tectibranchs, crawling with graceful elegance about the fronds of the waving Algæ, or floating at the surface of the still water in that reversed position already described.

Many more objects of like kind the observant naturalist will find from time to time, to gratify his curiosity and reward his diligence; twining Seaworms and Star-fishes; little Medusæ, like active bells of the clearest glass; the Beroë, a tiny ball of crystal; slender, shrub-like Zoophytes; and multitudes of other creatures, all shewing forth the glory of the great Workmaster, "for whose pleasure they are and were created."[2]


Family Aplysiadæ.

(Sea-Hares.)

In such rock-pools as I have just described, or among sea-weeds growing at low water-mark, a large fleshy Mollusk may occasionally be found, with two erect tentacles somewhat resembling the ears of the hare. It is the representative, the only British one, of the family Aplysiadæ.

The most prominent characteristics of this group are the following:—The mantle is greatly developed and dilated at the sides into large flexible crests, which can be turned up, and, surrounding the back on every side, can be reflected over it. The head is distinct, and separated from the body by a neck of greater or less length; its front forms a broad lip, drawn out at the corners into a pair of flattened tentacles; another pair is carried erect on the top of the head. The gills, in the form of complicated leaflets, are placed upon the back, and are generally covered with a convex, horny, or shelly plate, irregular, and varying in shape.

The Sea-hares are vegetable feeders, and, by a curious analogy with the herbivorous Mammalia, the digestive apparatus is highly complicated. According to Professor Grant, there are three stomachs; a short narrow gullet dilates into a large membranous crop; a curved bag, which is generally found filled with pieces of coarse sea-weed. This large crop or paunch occupies the right side of the

Natural History - Mollusca - Sea-hare anatomy.png

INTERNAL ANATOMY OF SEA-HARE.

body, and opens laterally into the middle stomach, which is the smallest of all, and performs the part of the gizzard. Its coats are thickened; and the interior callous lining is beset with firm, horny processes, in the form of rhomboidal plates or molar teeth, which serve to compress the softened vegetable matter transmitted in small portions from the first stomach. The third cavity of this complex apparatus is placed on the left side of the body; its interior surface is studded with sharp, horny spines, resembling canine teeth, to pierce and subdivide the coarse food, and thus prepare it for the action of the gastric juice, and other fluids accessory to digestion, which enter the stomach from adjacent organs.

Natural History - Mollusca - Sea-hare stomachs.png

SECOND AND THIRD STOMACHS OF SEA-HARE LAID OPEN.

The complexity of this structure has reference to the coarseness of the materials on which the animal subsists—the leathery fronds of the olive sea- weeds, which slowly and with difficulty yield their nutritive elements to the digestive functions.

The circulation of the blood in these animals has been considered, on the high authority of Cuvier, to present extraordinary peculiarities. The large vessel which collects and receives the venous blood from all parts of the system, and carries it forward to the gills, was believed to have numerous perforations in its walls, through which a free communication existed with the general cavity of the abdomen, so that the fluids contained in the one could readily permeate the other.[3] But it has been since proved that these supposed perforations are merely depressions, and that the lining membrane of this great blood-vessel is entire, as in other animals.[4]

The Mollusca of this Order undergo a metamorphosis exactly similar to that already described in the Nudibranchs; there is, in fact, scarcely any appreciable difference in the form of the newly-hatched young in either of these Orders, in that of the Pectinibranchs, and in the Class Pteropoda. How long the infant animal remains in this, its first condition, is not yet ascertained. Arrived at the second stage, we find it still enclosed in its transparent and nautilus-like shell; but the mantle has become detached, and covers tightly the mass of the viscera. The foot is so enlarged, that it forms a considerable projection beyond the margin of its operculum; and the veils have also grown in size, while the eyes have altered to a violet colour. The head has now two short, conical, ciliated tentacles, and the little animal swims with surprising quickness.

In the third stage the shell has fallen off, and the general shape is that of the parent, but the veils still remain. In the fourth stage, the creature begins to crawl in the gasteropod fashion, and the branchiæ and cæca begin to sprout. There are now, also, visible pulsations in the heart; and the mouth is armed with jaws, and with a spinous tongue. Another stage is marked by the fall of the veils, and by the budding forth of the anterior tentacula, as well as of the branchiæ; and the full evolution of these organs completes the metamorphosis and entitles the animal to the privileges of maturity.[5]


Genus Aplysia.

Most of the characters which distinguish this genus have been already enumerated in those of the family. The peculiarities by which it is separated from its fellow genera are chiefly the presence of an internal shell, and the position of the gill-plume. The latter organ assumes the form of complex leaflets, attached to a broad membranous footstalk, and concealed beneath the shell.

The genus is truly marine; yet M. Rang has observed specimens of A. dolabrifera inhabiting marshes in the island of Bourbon, where the water was almost fresh, and where Neritina and Melania, both essentially fresh-water genera, were its companions.

They swim freely, by means of the large fin-like expansions of the mantle, which are waved with an undulating motion. They have been seen also floating at the surface, suspended like Pond-snails from the inverted foot.

All the species, I believe, are remarkable for the power of pouring out, in copious profusion, a fluid of a brilliant purple hue, which readily diffuses itself through the surrounding water. I have already mentioned my own experience of this phenomenon in a West Indian species, and Mr. Patterson thus speaks of it in our native species, A. hybrida. "The first which our dredge brought up was placed on one of the rowing benches of the boat, and in a very short time emitted a rich purplish fluid, so copiously, that it ran along the board. Being transferred to a phial of sea-water, the purple dye was still given off in such abundance that the creature soon became indiscernible. It was not until the water was again changed that we had the opportunity of observing the ease and grace with which it moved about, elevating and depressing its mantle, altering the outline of its body, and extending and retracting its tentacula so incessantly, that an artist would have found a difficulty in catching its characteristic figure."[6] This fluid is said by Professor Goodsir to be secreted by the edge and internal surface. The secreting surface of the mantle consists of an arrangement of special nucleated cells, which are distended with a dark purple matter.[7]

Besides the purple secretion, the Sea-hares occasionally discharge, from an orifice situated behind the oviduct, a milky fluid highly acrid, and probably containing stinging thread capsules similar to those already described in Eolis. The Seahares have in all ages sustained the imputation of being highly offensive and injurious to man, and though in modern works it has been the custom to ridicule the charge, there is reason to think it may not be altogether groundless. Barbut declares that a sailor, in the Mediterranean, happening to take hold of an Aplysia, it gave him such instantaneous and excruciating pain, as to cause inflammation and the loss of his arm. A better authority, Mr. Charles Darwin, found a species at St. Jago, the secretion from which caused a sharp stinging sensation similar to that produced by the Physalia or Portuguese man of war.[8]

The account given by Bohadtch is remarkably clear and circumstantial. He tells us that the Lernæa (A. leporina) abounds in the Bay of Naples, where the fishermen excused themselves for not bringing it to him, saying it was a filthy thing which stank abominably. When removed from the sea, and placed in a vessel, there exuded a large quantity of a limpid, somewhat mucilaginous fluid, exhaling a sweetish, sickening, peculiar smell: but besides this, and distinct from its purple secretion, the Aplysia excretes also a milky liquor, formed in an internal conglomerate gland, which seems to be analogous to the kidney of vertebrate animals. As often as he took the Aplysia from the vase of sea-water, and placed it on a plate with the view of more narrowly examining its structure, the room was filled with a most fœtid, nauseous odour, compelling his wife and brother to leave the room, lest sickness and vomiting should follow. He himself could scarcely endure it, and during the examination had repeatedly to go out and breathe a purer air. His hands and cheeks swelled after handling the creature for any length of time, and as often as it ejaculated its milky secretion; but he is uncertain whether the swelling of the face proceeded from the halitus merely, or from having accidentally touched it with the hand besmeared with the liquid: probably the latter was the real cause, for when he purposely applied some of it to the chin, some hairs fell from the part.[9]

For myself, however, I can state, that I have freely handled many specimens of A. hybrida, both young and adult, when newly taken out of the water, without perceiving the least unpleasant sensation, either of feeling or smell.

Natural History - Mollusca - Sea-hare.png

SEA-HARE.

Among the ancient Romans, the Sea-hare was esteemed highly venomous, and it certainly formed one ingredient in the poisonous draughts that were used in the corrupt ages of the empire for the vile purpose of assassination. Locusta used it to destroy such as were inimical to Nero; it entered into the fatal potion which she prepared for the tyrant himself, and which he had not resolution to swallow; and Domitian was accused of having given it to his brother Titus. To search after the Sea-hare was to render one's self suspected; and when Apuleius was accused of magic, because forsooth he had induced a rich widow to marry him, the principal proof against him was that he had hired the fishermen to procure him this fearful animal. He averred, however, that his only object in procuring the Sea-hare was the gratification of a laudable curiosity.

Our native species (A. hybrida) is about three inches in length, of an olive or dark green hue, often marked with dark rings enclosing white areas; the mantle is sometimes clouded with purple or blue.

Family Bulladæ.

The shell, which in the preceding families is thin, small, and rudimentary, is in this family much more developed. The spiral character is distinct, and the general form and texture show a considerable approach to that condition in which it is more familiar to us, viz. that of an ample, turbinated covering, for the inhabitation of the animal, of stony hardness. Yet in none of the genera of this family does the shell perform the function of a dwelling-house for the animal: it is still more or less concealed by the flesh; not, indeed, imbedded in the substance of the mantle, but invested more or less completely by fleshy lobes or wing-like expansions, that turn up on each side and embrace it. In the genus Scaphander, however, which includes the largest British species of the family, the shell is entirely exposed, the wing-like lobes being smaller than usual.

In its texture the shell is generally thin, pellucid, and colourless, or nearly so, though in some of the species in which the family characters are beginning to disappear, pale colours are displayed by the shell, and its texture is stony. Its form is more or less rolled upon itself, but without a salient or produced spire, and the mouth is usually ample and widely expanded.

The animal is large, fleshy, and often slimy. The gills are concealed, the tentacles are so much shortened, widened, and separated, that their form is well-nigh obliterated, and they constitute a large square fleshy veil, beneath which are placed the eyes. The stomach is complicated, and in some species there is a shelly gizzard, with a peculiar grinding apparatus of great strength, needful for the demolition of the shells of other species of Mollusca on which the Bulladæ feed. They are very voracious, and sometimes swallow bivalves so large as quite to distort their own form, and render it almost unrecognisable. The shelly gizzard of Scaphander lignarius, a large species not uncommon on our coasts, was described by Gioeni, a Sicilian naturalist, as a new genus of multivalve shells, to which he gave his own name, calling it Gioenia. He even went so far as to describe the habits of the pretended animal, which was actually received into the catalogues of science by some of the most eminent names in conchology, until at length the imposture was detected and exposed.

The animals of this family generally inhabit deep water, that is to say, below the range of low tide. Occasionally, however, they stray within tide-marks.

Genus Philine.

In this genus the shell is thin, fragile, and colourless, of a broad rounded outline, with the aperture very wide, and a small spire, frequently concealed. The animal is proportionally large, slug-like, and slimy, with the power of secreting and of throwing off an adhesive mucus in copious abundance. The shell is partially covered by the mouth, the side lobes of which are well developed. The head disk is obscurely four-sided, without eyes or distinct tentacles.


THE GAPING BULLA.
Of the six species of this genus which are found in the British seas, the largest is P. aperta, the Gaping Bulla. It is an unpleasing, almost shapeless slug, very soft and slimy to the touch, of an opaque white hue, sometimes tinged with pale orange. Looked at from above, it appears to be composed of four portions,—the square head-disk, the body partly enclosing the shell, and the lobes or wings of the mantle turned up on each side and investing it. It is usually about an inch and a quarter in length, but individuals are found of a larger size.

The shell, on being dislodged, is transparent and colourless, but on drying loses somewhat of its clearness, and becomes of a lustrous white hue. Its surface is smooth, except for the concentric lines, which mark its progressive increase. The aperture is so open as to display the whole interior even to the summit.

The Gaping Bulla is common on our coasts, especially in the south, inhabiting muddy and sandy ground at a few fathoms' depth. I have dredged it abundantly in Weymouth Bay, a mile or two from shore. In captivity it is sluggish, remaining inert at the bottom of the vessel in which it is kept, so as to be an uninteresting subject of observation; added to this it continually secretes and discharges a viscid slime, which stretches through the water in long tenacious strings, and envelopes and distresses any other marine animals which may be its fellow-captives. Some of the Nudibranchs discharge mucus in the same manner; but I know of none that possesses the disagreeable quality to the same extent as this Bulla.