GIANT CLAM-SHELLS ON THE GREAT BARRIER REEF, AS EXPOSED AT EXTREME LOW TIDE.
The shell in the foreground is 4 feet in diameter.
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.
Photo by W. Saville-Kent, F.Z.S.] [Milford-on-Sea.
HIGHLY MAGNIFIED TONGUE OF A SEA-SNAIL.
With this structure its owner bores or files a hole through the shells of other molluscs upon which it preys.
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.
