The Living Animals of the World, Volume II: Birds, Reptiles and Amphibians, Fishes, Jointed Animals, Molluscs, etc.
The NAKED-GILLED SEA-SLUGS constitute a second clearly defined group. All the species ore essentially marine, and most abundant among seaweeds and coral-growths, over twenty
Photos by W. Saville-Kent, F.Z.S.] [Milfonl-on-Sea.
LIMPETS, WHELKS, AND BARNACLES. SHELL BEACH, ABROLHOS ISLANDS.
On the rocks at Ilfracombe when the tide was low. Composed of iridescent top-shells.
Photo* by W. SamlU-Kent, F.Z.S.] IMiljord-on-Sea.
GREAT CLAM-SHELL ON CORAL-REEF. GIANT CLAM-SHELLS ON THE GREAT BARRIER REEF.
AS EXP SED AT EXTREME LOW TIDE,The soft parts have been removed to show the size of the shell and the pure whiteness of the interior. The shell in the foreground is 4 feet in diameter. 741
of the World 742 The Living Animals
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. En 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
Photo by W. Saviile-Kcnt, F.Z.S.] [Milford-on-Sea. source include the ponderous HELMET-SHELLS, or
HIGHLY MAGNIFIED TONGUE OP A SEA-SNAIL. CONCHS, employed for the manufacture of cameos; the GIANT WHELKS and TRUMPET-SHELLS, often With this structure its owner bores or files a hole through the shells of other molluscs upon which it preys. 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
Shellfish, or Molluscs 743
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 Photo by W. Saville-Kent, F.Z.S.] [Milford-on-Sea. A SCORPION-SHELL. " Pelican's Foot," found on the British coast.instanced by the foregoing forms, contribute Closely allied to the largely to the world's commissariat, they also yield the much-prized material known as " mother-of-pearl " and the purest and most aesthetically 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 , such as the CEYLONmussels, occur naturally in banks and beds of vast extent. In some species, it is technicallyPEARL-OYSTER, the shell is small, and the mother-of-pearl substance, or " nacre," as 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 . and most valuable mother-of-pearl, are obtained. Pearlssize, which produce the thickest its fishery isof the best quality are more rarely found in this description of shell, and prosecuted primarily on account of the substantial substance and magnificent quality of its its adult state to a weight ofnacre. A single pair of shells of this species will attain in from 12 to 18 Ibs. 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 Eeef, 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 of a colony of these monsterperiods during abnormally low spring tides. A photograph 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
PhotobyW.Saville.Kent,F.Z.S.] [Milford-on-Sea. exposed to view when the EOCK-OYSTERS. shell-valves are partly open, This is the ordinary commercial oyster of the Australian shores. are brilliantly tinted.
of the World744 The Living Animals
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
Photo by W. SaviUe-Kent, F.Z.S.] [Milford-on-Sea. round the margin of the A QUEENSLAND OYSTER-BANK. mantle. Illustrating another characteristic growth-phase of the Australian rock-oyster. 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 itshundred species. While insipid and of no account for the human commissariat, many of 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.
CHAPTER II.
LAMP-SHELLS. as near allies of the Oysters \ LITTLE group of double-shelled creatures, formerly regarded J~. and Mussels, are the LAMP-SHELLS. Their scientific appellation, signifying " arm footed," relates to the two spirally convoluted arm-like structures which constitute the salient features in these animals. That of lamp-shells bears reference to the small circular perforation near the extremity, or " beak," of the united shells, which imparts to the entire structure a its sub-terminalnot altogether remote resemblance to an ancient Greek or Roman lamp with wick-hole. In all essential points of their organisation the lamp-shells differ so essentially from ordinary bivalves that they are now generally recognised as representing an independent animal class, having, as a matter of fact, a closer relationship with Worms than with Molluscs. In their earlier condition certain lamp-shells are indistinguishable from larval worms; while the convoluted arms of the adult animals, thickly beset with bristles and hairs, closely resemble