A. H. Cooke

“ The popular names of ‘shells,’ ‘shell-fish,’ and the like, as commonly applied to the Mollusca, the intrinsic beauty and grace of the shells themselves, resulting in the passion for their collection, their durability and ease of preservation, as compared with the non-testaceous portion,—all these considerations tend to unduly exalt the value of the shell as part of the organism as a whole, and to obscure the truth that the shell is by no means the most important of the organs. ”

“ Thus it may well happen that a visitor to a tropical island, Ceylon for instance, or one of the Greater Antilles, if he times his visit to coincide with the rainless season, may be grievously disappointed at what seems its unaccountable poverty in land Mollusca. But as soon as the weather breaks, and the moisture penetrates their retreats, every bush and every stone, in favoured localities, will be alive with interesting species. ”

“ There is therefore a marked contrast between the Mollusca of the tropics and of the temperate zones, while different regions in the same latitude, whether within or without the tropics, often show great diversity in their fauna. Every region is thus characterised by its Mollusca. The Mollusca, for instance, of Australia or of South Africa characterise those countries quite as much as do the kangaroo and the emu, the hartebeest and the ostrich; there is nothing like them anywhere else in the world. ”

“ On the liquid mud-flats to the north glide about Marginella, Nassa, and Truncaria, in the clean sandstretch to the west Olivella ploughs about by hundreds with several species of Natica, and Tellina and Donax bury themselves deep, while farther down are Artemis, Chione, and, where mud begins to mix with the sand, Mytilus and more Arca. Each of these species has its own habitat, often circumscribed to a few square feet at the most, and it would be utterly useless to seek for it anywhere except in its own special domain. ”

“ The ancestry of that fauna must be looked for, as far as the Gasteropoda are concerned, in the littoral and estuarine species; for the Pelecypoda, in the estuarine alone. The effect of the recess of the tide, in the one case, and the effect of the reduced percentage of salt, in the other, has tended to produce a gradual adaptation to new surroundings, an adaptation which becomes more and more perfect. It may be safely asserted that no marine species could pass into a land or fresh-water species except after a period, more or less prolonged, of littoral or estuarine existence. ”

“ And when we come to examine the land and fresh-water Mollusca, it is found not merely that some Mollusca are terrestrial and others fluviatile, but that certain species haunt the hills and others the valleys, some the recesses of woods and others the open meadow sides, some prefer the limestone rocks, others the sandy or clayey districts, some live only in still or gently moving waters, while others are never found except where the current is rapid and powerful. ”

“ The common limpet is the favourite food of the oyster-catcher, whose strong bill, with its flattened end, is admirably calculated to dislodge the limpet from its seat on the rock. When the limpet is young, the bird swallows shell and all, and it has been calculated that a single flock of oyster-catchers, frequenting a small Scotch loch, must consume hundreds of [57] thousands of limpets in the course of a single year. Rats are exceedingly fond of limpets, whose shells are frequently found in heaps at the mouth of rat holes, especially where a cliff shelves gradually towards a rocky shore. ”

“ As soon as an oyster is taken out of the sea, it closes its shells, and keeps them closed until the shock of removal has passed away, or perhaps until the desirability of a fresh supply of water suggests itself. The men take advantage of this to exercise the oysters, removing them from the sea for longer and longer periods. In time this has the desired effect; the well-educated mollusc learns that it is hopeless to ‘open’ when out of the water, and so keeps his shell closed and his gills moist, and his general economy in good condition. ”

“ The principal modifications of the foot are due to its employment as a burrowing organ. In genera which burrow but slightly it is small and feebly developed, while in genera which habitually excavate, it becomes the largest and strongest organ of the body. At the same time it has a tendency to shift its position from the ventral to the anterior margin, accompanied by a corresponding narrowing of the shell, until it arrives at the position seen in Mollusca of the shape of Mya, Pholas, and Solen. In sedentary or attached genera, e.g. Pecten, Chama, Ostrea, the foot tends to become aborted. ”

“ The principal characteristic of the Nearctic Region is the remarkable poverty of its land Mollusca. No district in the world of equal extent is so poor in genera, while those which occur are generally of small size, with scarcely anything remarkable either in colouring or form. The elongated land shells (Clausilia, Buliminus) , so characteristic of Europe, are entirely wanting, but a few Bulimulus, of Neotropical origin, penetrate Texas, and from the same sources come a few species of Glandina (as far north as S. Carolina) , Holospira (Texas) , and Helicina. ”

“ The most important factor in the environment, as determining distribution, is the temperature of the water, which is probably to be regarded as affecting not so much the adult Mollusca as their ova; for the adult might possibly support life under conditions in which the ova would perish. It appears that a sudden change of temperature is the most effective barrier to distribution, [384] and may bring the range of a species to an almost instantaneous stop, while a very gradual change will allow it to extend its range very widely. ”

“ The range, not merely of genera, but even of individual species, is astonishingly wide. This is especially the case with regard to the Pulmonata and Pelecypoda. The genera Limnaea, Planorbis, Physa, Ancylus, Unio, and Cyclas are world-wide. Out of about ten genera [282] of fresh-water Mollusca in New Zealand, one of the most isolated districts known, only one is peculiar. In South Africa and the Antilles no genus is peculiar. In the latter case, this fact is remarkable, when we consider that the same sub-region has at least ten peculiar genera of operculate land Mollusca alone. ”

“ The Indo-Malay fauna spreads eastward from its metropolis, but has practically no westward extension, or only such as may be traced on the eastern coasts of Africa and the off-lying islands. There appears to exist no other case in the world where the metropolis of a fauna is so plainly indicated, or where it lies, not near the centre, but at one of the ends of the whole area of distribution.
Comparing the two sub-regions, the Chinese is distinguished by the great predominance of Helix, while in the Indo-Malay sub-region Nanina and the related genera are in the ascendancy. ”

“ The Hectocotylus Arm.—Perhaps the most remarkable feature in the sexual relations of all the Mollusca is the so-called hectocotylus of the Cephalopoda. In the great majority of the male Cephalopoda, one of the ‘arms,’ which is modified for the purpose in various ways and to a greater or less extent, becomes charged with spermatophores, and sometimes, during union, becomes detached and remains within the mantle of the female, preserving for some considerable time its power of movement. ”

“ On the dorsal side of the typical molluscan body, between the visceral sac and the shell, lies a duplicature of the integument, generally known as the mantle. The depending sides of the mantle, which are usually somewhat thickened, enclose between themselves and the body mass a chamber of varying size and shape, called the mantle cavity, which communicates freely with the external air or water, and encloses and furnishes a protection for the organ or organs of respiration. ”

“ The violence of the surf may beat its inhabitants in pieces, the retreat of the tide exposes them, not merely to innumerable enemies in the shape of predatory birds and beasts, but also to a change in the atmospheric medium by which they are surrounded. Hence, in all probability, have arisen the various forms of adaptation which are calculated to bring about the ‘survival of the fittest’; hence, to [2] narrow our point of view to the Mollusca, the development of hard shells, or exoskeletons, hence the sand-burrowing, rock-boring, rock-clinging instincts of various genera and species. ”

“ According to Nordenskjöld, when the eider duck of Spitzbergen has only one or two eggs in its nest, it places a shell of Buccinum glaciale beside them. The appropriation of old shells by hermit-crabs is a familiar sight all over the world. Perhaps it is most striking in the tropics, where it is really startling, at first experience, to meet—as I have done—a large Cassis or Turbo, walking about in a wood or on a hill side at considerable distances from the sea. A Gephyrean (Phascolion strombi) habitually establishes itself in the discarded shells of marine Mollusca. ”

“ In this vast extent of land two distinct centres of influence are prominent—the Indian and the Chinese. Each is of marked individuality, but they differ in this essential point, that while the Chinese element is decidedly restricted in area, being, in fact, more or less confined to China itself and the adjacent islands, the Indian element, on the other hand, extends far beyond continental Asia, and embraces all the Malay islands to their farthest eastward extent, until it becomes overpowered by the Papuan and Australian fauna. ”

“ By placing the egg on a looking-glass the act of exclusion can be perfectly observed. For several days the inmate can be seen in motion, until at last a small crack appears in the surface of the shell: this gradually enlarges, until the baby slug is able to crawl out, although it not unfrequently backs into the shell again, as if unwilling to risk itself in the world. When it once begins to crawl freely, it buries itself in the ground for 4 or 5 days without food, after which time it emerges, nearly double its original size. ”

“ At the same time it must not be forgotten that the old systems of classification, which were based almost entirely on indications drawn from the shell alone, have been strangely little disturbed by the new principles of arrangement, which depend mainly on structural points in the animal. This fact only tends to emphasise the truth that the shell and animal are in the closest possible connexion, and that the shell is a living part of the organism, and is equally sensitive to external influences. ”

“ Green oysters are not always green, it is only after they are placed in the ‘claires,’ or fattening ponds, that they acquire the hue; they never occur in the open sea. The green colour does not extend over the whole animal, but is found only in the branchiae and labial tentacles, which are of a deep blue-green. Various theories have been started to explain the ‘greening’ of the mollusc; the presence of copper in the tanks, the chlorophyll of marine algae, an overgrowth of some parasite, a disease akin to liver complaint, have all found their advocates. ”

“ Had ‘Atlantis’ ever existed, we should have expected to find a considerable proportion of the Mollusca common to all the groups, and perhaps to Europe as well, and there would apparently be no reason why a genus which occurred in one group should not occur in all. As a fact, we find the species extremely localised throughout, and genera occur and fail to occur in a particular group without any obvious reason. ”

“ Roughly speaking, the line of demarcation follows the mountain ranges which separate Burmese from Chinese territory, but the debatable ground is of wide extent, and Yunnan, the first Chinese province over the border, has many species common with Upper Burmah.
The gigantic ranges of mountains which bound the sub-region to the north-west and north limit the extension of the Indian fauna in those directions in a most decisive manner. There is no quarter of the world, even in W. America, where a mountain chain has equal effect in barring back a fauna. ”

“ The process of digestion, as the food passes from one end of the tract to the other, is performed by the aid of the secretions of various glands, which open into the alimentary canal at different points in its course. The principal of these are the salivary glands, situated on the pharynx and oesophagus, and the liver, biliary or hepatic gland, connecting with the stomach. With these may be considered the anal and ink-glands, which, in certain genera, connect with the terminal portion of the rectum. ”

“ It is very remarkable to find that increased elaboration in the structure of the eyes does not necessarily carry with it increased sensitiveness, i.e. higher visual powers. Avicula, which is only provided with a few scattered ommatidia, which would entirely escape the notice of any one who had not seen them better developed elsewhere, was considerably more sensitive to light and shade than Arca, with its eyes of conspicuous size and much more perfect organisation, instantly contracting the mantle upon the impact of a shadow so faint as to be invisible to the experimenter. ”

“ Slowly the extended stomach returns to its place, and the monster settles back to an uncomfortable after-dinner siesta on an exposed boulder; for the starfish wraps its turned-out stomach around the prey it has secured, in place of attempting to devour the limey covering in which most of its game is protected. Once the mouth of the shell is enclosed in the stomach of the starfish, the creature soon sickens, the hinge-spring relaxes its hold, and the shell opening permits the starfish to suck out the gelatinous contents, and cast free the calcareous skeleton. ”

“ Some shells are brightly coloured, as, for instance, the various species of Cistella which live on the coralline rock in the Mediterranean; these exhibit bands or rays of alternate orange and bright pink. On the other hand, the shells of Terebratula vitrea are of a slightly translucent white, and of the utmost delicacy. They are very large, so that the cavity within the valves is of much greater size than the body of the animal, but in other genera the soft parts are packed very closely, and there is but a very small mantle-cavity or space within the shell unoccupied by the body of the animal. ”

“ The hoe we carry is thrust out and the mass dragged shorewards, when the rascal disgorges two large dog-whelks he has been in the process of devouring. We feel a comfortable glow of satisfaction to think that this enemy of our oyster-beds is also the enemy of our other enemy, this carnivorous borer. ”

“ Ocean currents, rivers, floods, cyclonic storms of wind, birds, and even beetles and frogs, play a part, more or less considerable, in carrying living Mollusca or their ova, either separately or in connexion with floating débris of every kind, to a distance from their native home. Accidental locomotion, of one or other of these kinds, combined with the well-known tenacity of life in many species (p. 37) , may have contributed to enlarge the area of distribution in many cases, especially in the tropics, where the forces of nature are more vigorous than in our latitudes. ”

“ A family of operculates, the Pleuroceridae, with 10 genera and about 450 species, is quite peculiar, a few stragglers only reaching Central America and the Antilles. The nucleus of their distribution is the Upper Tennessee River with its branches, and the Coosa River. They appear to dislike the neighbourhood of the sea, and are never found numerously within 100 miles of it. They adhere to stones in rapid water, and differ from the Melaniidae of the Old World and of S. America in the absence of a fringe to the mantle and in being oviparous. ”