CHAPTER I.

_SHELL-FISH, OR MOLLUSCS._

The Molluscan Group or Sub-kingdom represents one, if not the most important, of the invertebrate sections of living animals with relation both to its numbers and variety and in its commercial and economic utility to mankind. In its ranks are included all those animals generally known as Shell-fish, and familiar to the non-scientific in the shape of Oysters, Mussels, Whelks, Periwinkles, and the innumerable varieties of gorgeous or delicately tinted shells of tropical seas.

Collectively, Molluscs differ from all such invertebrate groups as Insects, Crustaceans, and Worms in that they possess neither jointed limbs nor jointed bodies, their body-substance being enclosed by a more or less distinct muscular sac, or integument, technically known as the "mantle." Molluscs possess no internal skeleton; but for the protection of their soft and otherwise defenceless bodies the mantle is among the great majority of species endowed with the property of secreting a more or less indurated calcareous shell, within which, when danger threatens, the creature can entirely withdraw. In some species the shell secreted is relatively small, and serves only as a protective shield to especially vital areas; while in a third very considerable assemblage a shell is altogether absent. The minute yet technically recognisable structural differences between the shells of even the most closely allied specific forms, and the wider and distinctly evident divergences that separate the more remotely connected varieties, furnish the basis for their classification and nomenclature by the systematic conchologist. Molluscan shells, being so extensively preserved in the fossil state, furnish the geologist with invaluable data for his determination of the age and respective relationship of the fossil-bearing strata of the earth's crust.

Having no jointed limbs, molluscs are dependent upon some other mechanical adaptation for their powers of locomotion. This, in the majority of species, is represented by a modification of the lower surface of the animal's body, which is so richly supplied with muscular tissues as to constitute an effective creeping-base. As a locomotive organ this muscular area is usually known as the "foot."

Of living molluscs some 50,000 distinct species have been recorded. The great majority of these organisms are, as is well known, marine. A very considerable number, however, are inhabitants of fresh-water; while a yet smaller proportion, like the Slugs and Snails and their allies, are especially adapted for a terrestrial existence. Excepting two relatively small and inconspicuous groups, the great natural division or sub-kingdom of Molluscs is separated by systematic zoologists into three main sections or classes. The particular modification of the locomotive organ, or foot, serves, on the one hand, to readily distinguish the first or most highly organised group from the second or central class; while the third or lowest one is as clearly separated from the second and first by the character of the shell. The first and most highly developed section includes such species as the Octopus, the Cuttle-fish, the Squid, and the several varieties of Nautiluses; to the second or central group are referred all the marine and terrestrial Slugs and Snails with their innumerable modifications; while the third and lowest group comprises all the double-shelled or bivalve forms, such as Oysters and Mussels.

In the OCTOPUS and its allies the creeping-base, or foot, is prolonged round its margin into eight or more arm-like extensions. The anterior of these in the earlier phases of their development grow round and enclose the mouth, which consequently opens upon the centre of the locomotive surface. The arms of the octopus and other forms are in most instances provided throughout their length with complex sucking-disks, wherewith their owners can seize and retain their prey or walk with ease and celerity over the smoothest perpendicular or overhanging rock-surfaces. The octopus, as the type of its class, possesses yet another most effective locomotive organ. This is the so-called funnel, or siphuncle, a membranous tube connected with the capacious gill-cavity which is formed by a folding of the mantle on the under-surface. When at rest or moving leisurely, the water taken in through the pocket-like entrance to this cavity is discharged through the funnel without any particular effort. The animal can, however, at will leave go its attachment to the rocks, and propel itself swiftly through water by successive forcible expulsions of the water through the funnel. By directing the aperture of the funnel to the right or left, the creature can also direct its course in whatever direction it desires. When thus swimming, its translation is necessarily backwards. Another notable feature of the octopus is the "ink-bag," a huge gland secreting an inky-black fluid, which, as produced by the common cuttle-fish, constitutes the sepia of commerce. The contents of the ink-bag are discharged through the funnel at the will of the animal; as soon as the ink is brought into contact with the water, it becomes distributed through it in the form of a thick cloud, under cover of which the mollusc makes good its escape from any attacking enemy.

The octopus in British seas by no means attains to its maximum growth. Examples with arms from 2 to 2½ feet in length are accounted large specimens. In Mediterranean waters, however, these dimensions are much exceeded, individuals with arms 5 feet long, which are capable of covering a circular area no less than 10 feet in diameter with their fully extended appendages, being frequently recorded. In the West Indies, on the North-west American coast, and also in Chinese seas, similar, if not larger dimensions are attained by these creatures. That these monster octopods, or "devil-fish," as they are sometimes designated, prove a source of danger to human life has been abundantly demonstrated. Lurking, as is their custom, among rock-crevices, they seize hold of any moving object which approaches within reach of their extended arms. Bathers in this manner have been seized and drowned, it being impossible for even the strongest swimmer to free himself from the clutches of one of these animals, which, while retaining a firm hold on the rocks with a portion of its hundred-suckered arms, has entwined the others around its victim.

The natural food of the octopus are crabs, lobsters, and their like; and in places like the Channel Islands, where the tide retires very low, leaving the rock-fissures inhabited by the molluscs more or less exposed, their presence may be often foretold by the accumulation of empty, broken-up crab-shells around the entrances to their retreats. In common with other members of its tribe, the octopus is furnished with a strong, horny, parrot-like beak, wherewith it can with ease break through the shells of its accustomed food. The year 1900 was remarkable for the extraordinary abundance of the octopus on the English south coast, the result of their collective depredations very seriously affecting the local crab and lobster fisheries. The pots laid down over-night, in place of yielding the following morning their customary quota of marketable crustaceans, were more often than otherwise found to contain nothing but broken-up shells and a loathsome "devil-fish."

The SQUIDS and CUTTLE-FISHES, with their large lustrous eyes, are especially adapted for an open sea life, and for this purpose are furnished with lateral fin-like membranous expansions. A more important structural distinction is their possession of two supplementary appendages, which, usually retracted within special pouches when not in use, can be shot out to a length at least twice that of the eight ordinary arms. Both the cuttle-fish and the squid, or calamary, are also the possessors of an internal calcareous or horny shell which underlies and strengthens the upper-surface. The cuttle-bone used as a dentifrice and ink-eraser is the product of the first-named mollusc. The Ten-armed group, as it is named, with reference to the two supplementary arms, ten in all, possessed by its members, is notable for including species whose dimensions not only exceed those of any other invertebrate type, but whose fully extended length rivals that of the largest vertebrates. Giant squids, or calamaries, have been taken off the coast of Newfoundland, yielding, with their tentacular arms extended, a linear measurement of over 50 feet, associated with an estimated weight of as much as 1,000 lbs. There can be no doubt that these giant squids have in many instances furnished the basis of the oft-recurrent sea-serpent stories, more especially on those occasions where the supposed marine reptile and a whale have been reported as seen engaged in combat. As a matter of fact the sperm-whales habitually feed on deep-sea squids, and have been known, when mortally wounded, to vomit forth detached portions of these gigantic molluscs. The long tentacular arms of one of these monsters, thrown around the whale with which it had entered upon a death-struggle, might at a little distance be easily mistaken for some huge snake-like organism.

The various species of NAUTILUS, including the so-called "Paper" and "Pearly" species, belong likewise to this group of Molluscs. In the former case, however, it is only the female animal which secretes a shell, and this is used as a cradle wherein she deposits her eggs and rears her young. The pretty romance of the PAPER-NAUTILUS, or ARGONAUT, as it is technically termed, floating on the sun-lit waves with spreading sails and an even keel, has unfortunately been entirely dissipated by the penetrating search-light of modern science. The animal only floats on the surface when ill, or when torn from its customary pasture-fields by abnormal storms, otherwise it creeps about the sea-bottom, or disports itself in the sub-marine grottoes like an ordinary octopus, with which, in point of fact, the shell-less male agrees in all essential details. The shell-cradle of the paper-nautilus is not vitally connected with the body of the animal, as is that of the pearly species and all ordinary shell-fish. It is freely detachable from the body, and during life is grasped and held closely to it by the expanded extremities of the two lateral tentacles by which the delicate shell is mainly secreted.

The next group comprises the great bulk of Simple-shelled Molluscs, of which it is impossible in these pages to give more than a brief enumeration of some of the most prominent. The Lung-breathing section, which is usually awarded the first place on the list, includes the familiar GARDEN-SNAILS, the SHELL-LESS SLUGS, the FRESH-WATER SNAILS which come to the surface to breathe, and many distinct terrestrial species. The largest living representative of this group is the huge Land-snail of tropical West Africa, sometimes known as the AGATE-SNAIL, the shell of which is not infrequently as much as 7½ inches long.

LIMPETS, WHELKS, AND BARNACLES.

On the rocks at Ilfracombe when the tide was low.]

GREAT CLAM-SHELL ON CORAL-REEF.

The soft parts have been removed to show the size of the shell and the pure whiteness of the interior.]

The NAKED-GILLED SEA-SLUGS constitute a second clearly defined group. All the species are essentially marine, and most abundant among seaweeds and coral-growths, over twenty species occurring in British seas. They are notable for the slug-like form of their body, which is usually supplemented by the outgrowth from it of complex, variously modified gill-filaments. In some species these external gills take the form of symmetrical flower-like tufts at the posterior end of the back, while in others simple or variously branched gills may be developed on the upper-surface. The colours of many of these sea-slugs are more brilliant than those of any other molluscs, this being especially the case with the tropical coral-reef-frequenting species. Bright scarlet, yellows, and blues, separately or variously combined, are among the dominant tints. Many of these tropical species are also of considerable size. One particular kind, having a flower-like dorsal gill-tuft, observed by the writer on the West Australian reefs, was over 10 inches long and 8 inches broad. Its general ground-colour was intense vermilion, relieved, however, by a frilled border nearly an inch in width of the purest white, with radiating streaks of scarlet. It is an interesting circumstance that these naked-gilled molluscs, shell-less so far as their adult phases are concerned, emerge from the egg with a perfectly formed, but necessarily very minute, transparent shell, resembling that of a garden-snail. It is consequently inferred that the group has been derived from some permanently shell-bearing form.

The Comb-gilled section embraces the great majority of the marine molluscs having a single more or less convoluted or spirally twisted shell. They take their name from the circumstance that the gills have a compactly disposed comb-like contour. This gill-tuft is situated in an excavated chamber inside the shell, immediately over the neck. The COMMON WHELK, the PELICAN'S-FOOT SHELL, and the WINKLE are a few typical British marine representatives of this group, which, however, attains to the zenith of its development in the size, variety of form, and ornate coloration of its shells in tropical seas. The inter-tropical coral-reefs in particular yield a most abundant harvest in this direction. The shells in common use obtained from such a source include the ponderous HELMET-SHELLS, or CONCHS, employed for the manufacture of cameos; the GIANT WHELKS and TRUMPET-SHELLS, often over 18 inches long, used as signal-horns throughout Polynesia and on the tropical Australian coast; and the capacious MELON-SHELLS, made to do duty for boat-baling and as water-vessels and general domestic receptacles throughout the same tropical area. To this list may be added the HARP-SHELLS, VOLUTES, CONES, MITRES, OLIVES, THORNY WOODCOCKS, and a host of others prized by the conchologist. To this section must also be referred the innumerable species of COWRIES, of which the large, boldly mottled "TIGER" and "PANTHER" species are well known. The comparatively small, yellowish, thickly built, porcelain-like shell of the "MONEY-COWRIE" constitutes, as is well known, the current coin throughout extensive areas of Africa and India. It is recorded that as large a quantity as sixty tons of these small shells, originally collected from tropical seas, have been shipped from one British port alone to the African coast for commercial use within a single year. One very diminutive cowrie, pale pink in colour, with a delicately streaked surface, is indigenous to British waters.

The third large group of Molluscs which demands attention is that of the Bivalves, or Leaf-gilled group. Though not so numerous in species as the last, it outrivals it in the enormous abundance in which the individuals of many varieties are produced. OYSTERS, MUSSELS, COCKLES, SCALLOPS, and other allied forms occur in closely associated colonies, constituting natural "beds" or "banks," which may be of vast extent and, in at any rate the case of oysters, several feet in thickness. From a commercial and economic standpoint this group is undoubtedly of the highest importance to the human race. Not only do its members, as instanced by the foregoing forms, contribute largely to the world's commissariat, they also yield the much-prized material known as "mother-of-pearl" and the purest and most æsthetically beautiful gems--orient pearls. Pearls and mother-of-pearl are the products of two groups of shell-fish, respectively known as PEARL-OYSTERS and PEARL-MUSSELS. There are a considerable number of species, mainly denizens of tropical seas, which, like ordinary oysters and mussels, occur naturally in banks and beds of vast extent. In some species, such as the CEYLON PEARL-OYSTER, the shell is small, and the mother-of-pearl substance, or "nacre," as it is technically termed, so thin as to be of relatively little value. Hence the fishery for this species is conducted almost exclusively for the sake of the pearls, which are fairly numerous and frequently of the finest quality. From the tropical Australian seas pearl-shells of the largest size, which produce the thickest and most valuable mother-of-pearl, are obtained. Pearls of the best quality are more rarely found in this description of shell, and its fishery is prosecuted primarily on account of the substantial substance and magnificent quality of its nacre. A single pair of shells of this species will attain in its adult state to a weight of from 12 to 18 lbs. The fishery for this pearl-shell has, however, been prosecuted so relentlessly that bivalves of such matured age and weight are now of rare occurrence, and obtained only from almost inaccessibly deep waters. Unless, in point of fact, systematic methods of conservation and cultivation are resorted to on an extensive scale and on lines corresponding fundamentally with those successfully followed in the culture of ordinary commercial oysters, there would seem to be an imminent risk of the valuable Australian pearl-shell fisheries becoming depleted to more or less complete exhaustion.

The tropical Australian seas, and notably those which wash the Great Barrier Reef, are famous for the production of the largest of living bivalve molluscs. These are represented by the GIANT CLAMS, which, dwelling among the coral-growths, are left exposed to view for brief periods during abnormally low spring tides. A photograph of a colony of these monster bivalves, taken by the writer amidst this mollusc's characteristic surroundings, is reproduced on page 741. The example in the foreground measured no less than 4 feet in diameter and weighed several hundred-weights. In many clams the living tissues, or mantle-borders, that are exposed to view when the shell-valves are partly open, are brilliantly tinted.

Of the SCALLOPS many of the larger species are highly esteemed for the table. It is notable of them that they can progress through the water in a jerky, flight-like manner by the repeated flapping of their opposing shells. In many scallops the jewel-like eyes are developed in a row round the margin of the mantle.

The Bivalve Class includes certain representatives which are held in evil repute, on account of their destructive habits. PHOLAS and its allies bore their way into solid rock, bringing about its complete disintegration. From the dreaded SHIP-WORM, or TEREDO, on the other hand, there would appear to be no description of wood that can withstand its ravages.

The Bivalve Molluscs are not without fresh-water representatives. The well-known POND- and RIVER-MUSSELS, which form the most conspicuous examples of this group, number several hundred species. While insipid and of no account for the human commissariat, many of its members produce pearls of value. One such, obtained from the river Conway, in North Wales, is said to occupy a place in the crown of England.

The CHITONS, or MULTIVALVE MOLLUSCS, invite brief notice. As the last-named title implies, the shell-elements in this group are relatively numerous, consisting of eight pieces, or plates, which may form contiguous transverse shelly shields that entirely cover in and protect the dorsal surface of the elongate, boat-shaped body, or may be more or less isolated from one another. In the former instance the animal bears some considerable resemblance to a gigantic limbless wood-louse, and, like that familiar terrestrial crustacean, it is capable of rolling itself into a spheroidal shape as a means of protection. None of the chitons are provided with eyes in the adult state in that region of their body--namely, the head--where they might be most naturally expected to exist. Recent scientific investigation has, however, elicited the fact that in various species the respective shell-plates are studded with minute eye-specks, the aggregate number of visual organs thus possessed by certain forms reaching to the astonishing figures of 11,000 or 12,000. The majority of the chitons are shallow-water, rock-frequenting molluscs, which may be successfully sought by turning over stones at low water. Several species are inhabitants of British seas.

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