Genus Limnoria
L. lignorum, the gribble. This little isopod is very destructive in its habits, boring into submerged woodwork, like the ship-worm (Teredo), and doing great damage to the piles of wharves, etc. It burrows to the depth of half an inch, and completely honeycombs the surface of the wood, which then scales off or rapidly decays, and is washed away by the waves. The animal then begins anew, and in a short time, or at the rate of about an inch a year, piles diminish in diameter where Limnoria is plentiful, and are finally destroyed unless protected from these little crustaceans by a sheathing of metal. Although considered as pests, they have an economic value not so often recognized, as they attack all driftwood, and in time entirely destroy floating and water-logged [pg293] timber, which without their destructive agency might become serious obstructions to navigation. Limnoria is only the fifth of an inch in length, and its back is covered with minute hairs, to which dirt usually adheres. It burrows with its mandibles, or jaws, which are chisel-like at the ends. Its habitat extends from a little above to a little below tide-marks throughout the whole Atlantic coast.
Genus Sphæroma
S. quadridentatum. The name of the genus to which this species belongs is derived from the peculiar habit of many of the species of rolling themselves into a ball when alarmed. The body is so constructed as to admit of this singular change of shape. The abdomen turns under, and the last abdominal appendages, together with the narrow epimera, shut in the legs and cover the ventral portions with armor. The body, when extended, is an ellipse a little over a quarter of an inch in length and half that in breadth. The legs are hairy and adapted to walking. The anterior abdominal segments are fused into one, but are marked at the sides with depressed lines. The abdominal feet are plate-like and fringed with hairs. A slight elevation runs around the margins of the animal like a border. The color is variable. Some are a uniform slaty gray; others are marked with a longitudinal patch of color on the back. It is found from Cape Cod to Florida among algæ or rocks, and is easily recognized by its habit of rolling itself into a ball. Sphæroma destructor is a boring isopod, larger than Limnoria, and is even more destructive, since its holes are larger; but its range is limited, or, at least, it has so far been found only in the St. Johns River, Florida.
Genus Idotea
I. marina (Linnæus), I. irrorata (Edwards). This species is about one inch long, and is easily recognized by the abdomen, the first three segments of which are narrow and terminate in acute teeth, while the other three are fused into one with straight sides and ending in three teeth, of which the middle one is the longest. The first pair of abdominal feet are large, long, and plate-like, covering the other feet and whole under surface of the abdomen like an operculum. The head is nearly square, the eyes are small, and the antennæ have long peduncles. The articulations of the thoracic feet are fringed with hair. The color of this species varies: sometimes it is light or dark green, or brown with black spots; again [pg294] it is striped longitudinally with light color. Sometimes the stripes run transversely in bands or spots. It is found from New Jersey northward on rocky shores of bays and sounds, or in eel-grass on sandy shores.
I. ochotensis. A species very similar to the preceding, but a little larger, found on the Pacific coast.
I. metallica. This species is a little smaller than I. marina, and differs from it in having the end of the telson truncated, or straight, instead of dentate. The head is nearly square. The body is broadest in the middle of the thorax; the projecting epimera give a serrate appearance to the sides. The abdominal feet are inclosed in the operculum-like scales of the first pair of feet. Its color is bright blue or green, often with a metallic luster when seen in the water. It is found swimming free or floating in masses of seaweeds from Long Island northward and also on the Pacific coast.
I. wosnesenskii. A common species on the California coast, about one inch long and dark in color. The abdominal segments are united and bluntly rounded, and each has a small median tooth. The abdominal feet are inclosed by the opercular feet. Eggs and young are carried in a pouch.
Genus Chiridotea
C. cæca. This species burrows beneath the surface of the sand, raising it up into a little ridge, and makes a mound at the end of the burrow, where it may be found. These trails are common on the wet sand. It imitates the sand in color, and is about half an inch in length, with a broad thorax and narrow abdomen terminating in a point. It is found on the New England coast.
C. entomon (Linnæus). This species of the northern Pacific coast is from one to three inches long. The head has the appearance of two lobes, the sides having two rounded ends. The body is broad, the abdomen narrowing to a pointed end. The last segments of the abdomen are united, and the abdominal feet are inclosed by the first pair, which extend over them like an operculum. The epimera are broad, with acute lateral angles.
ORDER XIPHOSURA
Genus Limulus
L. polyphemus, the horseshoe- or king-crab. This well-known and curious animal ranges along the Atlantic coast from Maine to Mexico. [pg295] It lives on sandy and muddy shores below low-water mark, where it burrows beneath the surface. At the breeding season—May, June, and July—it comes ashore to deposit its eggs near high-water mark. The crabs come up the beach in pairs, the male being the smaller and riding on the back of the female, holding on by short feet provided with nippers, which are peculiar to the males. Sometimes the female is accompanied by several males, each one holding on to the tail of another and forming a string of animals. After the female has deposited her eggs in a hole excavated by her for the purpose, the male covers them with milt, and they then return to the water, leaving the eggs to be buried in the sand by the action of the waves. The eggs hatch in July and August, and sometimes the beach is literally alive with the young crabs, which, however, soon disappear, and are not seen again until they are well grown. After the spawning season the adults are not very often seen, but usually their empty shells, abandoned in moulting, may be found on the shore. The horseshoe-crab, also called king-crab, is especially interesting from the fact that it is the last survivor of an otherwise extinct group of animals. Its relationships with classes which have become extinct, its nearest relatives being fossils, make it difficult to classify definitely with existing forms. Formerly it was regarded as a crustacean; now it is classed by some authors with the Arachnida, along with scorpions and spiders. It has the characteristics of both groups. There are only two known species of Limulus in the world; the other, Limulus moluccanus, lives on the eastern coast of Asia. L. polyphemus often measures a foot in diameter. The body is composed of three parts. The front portion, or cephalothorax, is broad and semicircular, with posterior angles ending in points. Near each side of its dorsal surface is a pair of large compound eyes covered with thickened cuticle, and near the center line of the shell, at the base of the first spine, a pair of small, simple eyes. The second portion is the abdomen, and the third a long, movable spine attached to the last segment of the abdomen and between two terminal points of the horn-like covering. [pg296] On the cephalothorax are seven pairs of appendages. The first pair are small and lie in front of the mouth; these and the four following pairs have chelæ, or claws. The sixth pair have no chelæ, but at the base have a peculiar process termed the flabellum. These six pairs of appendages surround the mouth, and have bristles at the base which serve as jaws. The seventh pair of thoracic appendages are broad plates called the operculum, which meet and cover the abdominal appendages to a certain extent. The abdomen has five pairs of appendages, which are plate-like and resemble the operculum in form and have an upper and an under piece. On the posterior surfaces of the abdominal feet are large, leaf-like folds, which are the gills. In front of the first pair of thoracic legs is a small tubercle supposed to have an olfactory function. Limulus has a blood-vascular system (the blood being bluish in color), a nervous and a digestive system. It walks with the thoracic feet and swims with the abdominal ones. It enters the sand by pushing in the rounded, anterior end. In moulting the carapace opens on the front edge. Limulus is edible, its meat being, it is said, as good as that of the lobster. (Plate LXIV.)
| PLATE LXIV. | |
|---|---|
| Calappa flamma. | Calappa flamma, claws open, showing form of cockscomb. |
| Lambrus pourtalesii. | Limulus polyphemus. |
ORDER PYCNOGONIDA
This order of animals, like Limulus, has no definitely determined position in the classification of marine organisms. About one hundred and fifty species of Pycnogonida have been described, of which thirty are found on our coast and are commonly known as sea-spiders. They are found crawling slowly over seaweeds, hydroids, and sponges, and appear to be all legs, but, like other animals, have a digestive, a circulatory, and a nervous system. They are devoid of organs of respiration. The body has a cephalothorax, which bears a conical suctorial proboscis on the anterior end, and on top a prominence containing four eyes. It has three pairs of appendages, two pairs of which are sometimes lacking; then come three free segments and a rudimentary abdomen. There is a pair of short appendages bearing claws (chelæ) and four pairs of long walking-legs. The walking-legs contain a tubular outgrowth of the body, into which the stomach extends. The egg-sacs are in the legs, and open at the basal joints. The male is provided with an extra pair of legs, for the purpose of carrying the eggs after they are deposited by the female. The [pg297] ovigerous legs are run through sac-like pockets and bent under the body, and thus the eggs are carried until hatched.
Genus Phoxichilidium
P. maxillare. This species is found on the New England coast creeping over hydroids and ascidians. It is purple, gray, or brown in color. Its young take shelter in the cavities of hydroids, forming galls.
Abductor muscles: Muscles which move parts away from the axis.
Adductor muscles: Muscles which draw parts together; opposite of abductor.
Aperture: Opening of the spiral shell.
Bilateral symmetry: Having two equal sides, divided by a central axis.
Branchiæ: Gills, or breathing-organs; organs subservient to respiration through water. Same as ctenidia.
Buccal mass: The mouth-parts as a whole; the organ of prehension and mastication of food; the pharynx: present in all mollusks except lamellibranchs.
Canal: A narrow prolongation of the aperture of a spiral shell.
Carinated: Ridged as if keeled; having a keel.
Cinereous: Ash-gray; having the color of wood-ashes.
Cirri, plural of Cirrus: Filamentous appendages.
Columella: An upright pillar in the center of most of the univalve shells, round which the whorls are convoluted.
Concrescent: Growing together; uniting.
Cordate: Heart-shaped.
Costæ, plural of Costa: Ridges of a shell.
Crenulated: Having a series of notches; marked as with notches, as the indented margin of a shell.
Crystalline style: A transparent gelatinous substance of unknown function, which fills, at times, the stomach-parts of certain mollusks.
Ctenidia, plural of Ctenidium: Gills, or breathing-organs, adapted to water-respiration. [pg303]
Decussated: Crossed; intersected.
Dextral: Having the aperture on the right side of the shell when the apex is upward.
Epidermis: The horny coating or outer skin of some shells.
Foot: The ventral surface of the body on which the animal rests or moves.
Fossette: A little hollow or pit.
Fuscous: Brown tinged with gray; swarthy.
Fusiform: Tapering both ways from the middle.
Lamelliform: Lamellate in structure; disposed in leaf-like layers.
Lingual ribbon: The chitinous band of teeth, or rasp, borne upon the odontophore; the radula.
Lunule: An impressed area just below the beaks of bivalve shells.
Mantle: A fleshy or membranous outgrowth of the outer body-wall; also called pallium.
Mantle cavity: The space between the mantle and the body.
Monomyarian: Having one adductor muscle, as an oyster.
Nephridium: The renal organ of mollusks, corresponding to kidneys in vertebrates.
Node: A knob or protuberance; also a notch in the margin.
Odon´tophore: The lingual ribbon bearing chitinous teeth.
Oper´culum: A horny or shelly plate which serves to close the aperture of the shell when the animal is retracted.
Osphra´dia, plural of Osphradium: Olfactory or water-testing organs.
Otocyst: The cavity, or cyst, which contains the essential parts of an organ of hearing.
Pallial line: The impression or mark made by the mantle, or pallium, on the inner surface of a bivalve shell.
Pallial sinus: A notch or recess of the pallial line; the scar of the siphon.
Papillaceous: Warty; studded with bosses; having excrescences.
Perios´tracum: Same as epidermis.
Per´istome: In zoölogy, mouth-parts in general; in conchology, the margin of the aperture of the shell. [pg304]
Porcelanous: Resembling porcelain.
Rad´ula: Same as lingual ribbon.
Reticulated: Having distinct lines or veins crossing like a network; covered with netted lines.
Sculpture: Elevated or impressed marks on the surface; markings resulting from irregularity of surface; tracery.
Sinistral: Having the aperture of the shell at the left; opposite of dextral.
Sinuate: Curved in and out; wavy.
Siphon: A tubular fold, or prolongation of the mantle forming a tube.
Spire: All the whorls of a spiral univalve, above the first large body-whorl.
Suture: The line of junction of the successive whorls of a univalve shell, or the line of closure of the opposite valves of a bivalve shell.
Teeth: Tooth-like projections on the hinge of a bivalve shell.
Testaceous: Having a hard shell, as distinguished from crustaceous, or soft-shelled.
Turbinate: Shaped like a whipping-top; whorled from a broad base to an apex.
Umbilicus: A central, navel-like depression in the body-whorl; a circular, more or less central pit or hollow.
Umbo: The beak of a bivalve shell; the protuberance of each valve above the hinge.
Varices, plural of Varix: Raised marks on the shell denoting a former position of the lip of the aperture, and indicating periods of rest.
Varicose: Having prominent and tortuous formations on the shell.
Ventricose: Having the whorls of the shell swollen or strongly convex.
Whorl: One of the turns of a spiral shell. Body-whorl, the last whorl.
We now come to another large and important group of animals, with characters so peculiar to itself and differing so radically from those which distinguish any other class of living creatures, that it may be said to occupy almost a unique position in the animal kingdom. Recent investigations have demonstrated that the larval form of mollusks presents some remarkable points of similarity to the embryonic forms of the Annelida and to the larvæ of some other classes of lower organisms. This discovery is one of the triumphs of embryology in its patient search for that connecting-thread that weaves together all the varying forms of animal life. Aside from these subtle evidences of relationship revealed by the microscope, the mollusks appear to occupy a position of considerable isolation in the biological world.
As accepted by zoölogists to-day, this phylum is but the remnant of its former self. Aristotle considered all creatures with a testaceous covering to belong to a single family, and those later patriarchs of biology, Linnæus, Cuvier, and Lamarck, extended the group to include the greater part of all the marine invertebrate animals. Little by little the phylum has been shorn of orders and classes. First, the worms and the Echinodermata were separated into distinct phyla; then the barnacles were discovered to be crustaceans, and were accordingly removed from their position as "multivalves" under the Mollusca; then the tunicates, or ascidians, were found, through the critical examination of their larval stage, to be merely masquerading as mollusks; and lastly, the brachiopods have been somewhat reluctantly removed from their old position with the mollusks and given the [pg306] distinction of a phylum almost exclusively to themselves: they retain their old associations only in the name of Molluscoida.
With some exceptions, all mollusks secrete from their outer skin, or mantle fold, a calcareous protective covering, or shell. This may be either "univalve" or "bivalve" according as it consists of one or two pieces. This phylum includes all the sea-shells which are so commonly found along every ocean beach, in the tide-pools, on rocks at low tide, in estuaries, and, indeed, wherever sea-water is present. The phylum also includes, as one of its large suborders, all the snails and slugs that are to be found crawling upon the land. These are true mollusks, which differ essentially from their marine brethren only in that they breathe by means of a pulmonary sac or lung instead of by gills. There are also many genera and species of mollusks that find their habitat only in the fresh water of rivers, lakes, or ponds; curiously enough, many of these fresh-water forms, like the purely terrestrial snails and slugs, are air-breathers, possessing no gills whatever, and are consequently obliged to make periodic visits to the surface of the water to obtain their necessary supply of oxygen. There are also numerous forms of mollusks that are entirely deprived of a shell covering; and, again, there are intermediate types between these two extremes that produce only more or less developed rudimentary shells. Notwithstanding these variations in the matter of a shell covering,—an important consideration in this phylum,—by reason of their anatomical features these "naked" forms are mollusks quite as much as are those that secrete the most highly developed tests.
The marvelous beauty of sea-shells and tropical land-shells, their almost infinite variety in form and coloring, has given to them an interest among collectors that is very great. There are many wonderful conchological collections in public museums and in private cabinets. It would well repay the lover of beauty as well as the more serious student of nature to examine carefully such collections when opportunity offers, for nowhere in the realm of nature can more exquisite coloring and modeling be found. The fact that shells may be preserved for all time without the expense and the vexations of preservative fluids has no doubt induced many [pg307] to choose this field of natural history for special study. But the true student of zoölogy does not confine his investigations in this phylum to considerations of the shell only, for the protective armor constitutes only a portion, although an important portion, of the anatomy of the mollusk. On the same principle, it would hardly suffice to examine only the skeleton of vertebrate animals in order to become acquainted with the fishes or mammals. In a sense, the testaceous covering of a mollusk may be likened to an outer skeleton.
The older zoölogists were inclined to ignore almost entirely the animal or "soft parts" of mollusks, and their classification of the phylum into "univalve," "bivalve," and "multivalve," based upon shell-characters alone, has since been proved to be wholly artificial. The terms "univalve" and "bivalve" are, of course, often and correctly used to-day to designate mollusks possessing a single or a double shell, such as the common garden-snail, which carries upon its back a single spiral shell, and, on the other hand, the oyster or clam, which is inclosed within two valves which fit closely together. But these terms are not technically used, although they do indicate in a general way the more modern classification. Even among some comparatively recent conchologists a tendency to belittle the value of the "slimy creature" within the shell has led to some curious errors of grouping—mistakes which more thorough and scientific investigation has brought to light. In general, the higher classification into classes, orders, suborders, and families is exclusively based upon anatomical features, and to a large extent, also, is that of the genera; but species are always determined by their shell or conchological characteristics.
When a certain familiarity with the forms and general appearance of shells is gained, a glance at the shell alone will almost always suffice to place it at once in its true generic position, for, along with modifications of the animal itself, corresponding modifications of the shell are quite certain to take place. Hence a fairly expert zoölogist should be able to determine with considerable accuracy from an empty and dead shell the anatomical features of the animal that at one time inhabited it. [pg308]
One of the main reasons for the special interest that seems to have attached to the study of conchology is derived from the fact that the durability of shells has caused their almost perfect preservation as fossils from the very earliest periods of geological time, thus furnishing the key to the solution of many problems of evolution. Upon this page of the earth's history the letters are sharp and clear, and geologists possessing a knowledge of recent forms of mollusks as well as of their geographical distribution have been enabled to read some wonderful stories of the cosmic history of islands and continents.
Mollusks are found in every part of the world. The arctic seas possess their own characteristic faunæ, the more temperate waters of Europe and of America contain their own peculiar genera and species, and the warm waters of the tropic seas furnish the conditions favorable to the life of an immense number of characteristic forms. Again, there are the littoral species, that live only between tide-marks and are therefore exposed to the air for a number of hours each day; some of these live just about high-tide mark and have become almost terrestrial in their habits, while others must be sought at the point of lowest tide, where for only a short time each day they are deprived of their natural element. Then there are the shallow-water forms, which never appear above low-tide mark save when a neap tide surprises them; their range in depth extends to about the hundred-fathom line, more or less. Beyond this depth to several hundred fathoms other characteristic forms appear, and from the more profound depths of mid-ocean the dredge has brought to light a host of curious and interesting species.
It is the temperature of the water rather than the depth that appears to influence the distribution of marine mollusks. Thus, certain species whose natural home is in the shallow waters of the Arctic Ocean have been taken in very deep waters off the southern coast of the United States, the temperature conditions in both stations being substantially the same. This fact, however, must not be too freely accepted as establishing a principle. Some [pg309] shallow-water forms have been taken at great depths, but in general the abyssal fauna is a peculiar one that cannot well be marked off into geographical provinces.
The east coast of America is divided into several molluscan faunal regions. A series of very-cold-water forms belonging to a circumpolar region, called the "arctic province," are found as far south as Newfoundland. On the New England coast a number of these arctic species are also found, urged south by the influence of the cold Labrador current. The most characteristic genera belonging to this arctic fauna which are found upon the Maine and Massachusetts coasts are Buccinum, Chrysodomus, Sipho, Trophon, Bela, Velutina, Trichotropis, Lacuna, Margarita, Pecten, Leda, Yoldia, Astarte, and Mya. Examples of all these genera are encountered as far south as Cape Cod.
A "boreal province" corresponds with a similar faunal region upon the European shore. The mollusks which compose this fauna are about three hundred in number, and range along the New England coast from the Gulf of St. Lawrence to Cape Cod. It is a somewhat remarkable fact that many of these species are identical with English and French forms. The striking genera upon the American side are Purpura, Littorina, Polynices (Lunatia and Neverita), Acmæa, Margarita, Chiton, Doris, Æolis, Mytilus, Modiola, Thracia, and Nucula.
Cape Cod has been regarded, until very recently, as a sharp divisional point between the boreal and the transatlantic provinces, the latter faunal area extending down the Atlantic coast of the United States to southern Florida. At Cape Cod the Labrador current is deflected from the coast, and the warmer shore waters south of that point are unfitted for the development of the boreal forms, though some of them, as we shall see, have passed the [pg310] barrier and maintain themselves very successfully at Martha's Vineyard and in Long Island Sound. Some of the species whose natural habitat is south of Cape Cod are also to be found north of that point, but they do not reach their full development in the colder waters of Massachusetts Bay.
The long stretch of coast-line from Cape Cod to Florida is easily susceptible of faunal subdivision at Hatteras, below which locality a decided mixture of Antillean species is apparent. Between Long Island Sound and Hatteras the littoral molluscan fauna is disappointing to the collector, for the number of species is relatively small, and few even of these are entitled to any claims to beauty of shell. The nature of this coast, virtually a great sweep of exposed sand-beach, is not conducive to a varied or rich fauna. But if this portion of the transatlantic province is lacking in interest to the shell-collector, it is anything but disappointing to the army of men employed in the oyster- and clam-fishery. It is only within this faunal area that the American oyster, Ostrea virginica, the best of the edible shell-fish, finds its natural home, and here the oyster-culture is most extensively carried on.
After storms the Jersey beaches are frequently strewn with Mactra, Tagelus, Arca, and Ensis; in more sheltered places Fulgur and Polynices (Lunatia) are commonly met. These may be accepted as the most characteristic genera of this province.
Just south of Cape Hatteras an observer is struck with the sudden change in the appearance of the shells on the beach. Cardium, Cassis, Dolium, Arca, and Cancellaria at once suggest the West Indies. About the vicinity of Hatteras the Gulf Stream approaches very near the land, bringing with its warm waters many wanderers from tropical homes. From Beaufort to Florida there is another long stretch of exposed and shifting sand-beach, which offers a scant return to the collector of mollusks.
When Florida is reached a new world is opened to the naturalist, for there a better acquaintance is made with the great Caribbean province, which, extending from Florida to the northern shores of South America, embraces all the Bahamas, the West [pg311] Indies, the Gulf of Mexico, and the coast of Central America. Strangely enough, the mollusks of the west coast of Florida are more nearly related to those of the transatlantic province than are the shells of the east Florida coast. This remarkable fact in geographical distribution can only be explained by assuming that an open waterway once existed across the northern part of the Florida peninsula, connecting the Atlantic and the Gulf, through which a mingling of the shallow-water forms from Georgia, the Carolinas, West Florida, and the Gulf coast took place. Thus it would appear that when Florida was an island the fauna of its east coast, originally largely Antillean, was less disturbed by the southern migration of American species. The Florida Keys are essentially Antillean. The Caribbean or Antillean fauna is an exceedingly rich and varied one. Among the most prominent genera which may be encountered upon the Florida coast are Strombus, Fasciolaria, Oliva, Marginella, Natica, Sigaretus, Littorina, Tectarius, Neritina, Melongena, Cardium, Callista, Tellina, Lucina, and Cyrena. In such an extensive and rich faunal province one must not expect to find in any given locality a representation of all the characteristic genera. For instance, collections made at Havana, at Nassau, and at Vera Cruz would each contain forms more or less peculiar and local, but certain species would be found in each collection.
On the west coast of America quite the same faunal division
into more or less well-marked provinces is to be found. Bering
Sea belongs to the arctic province, and contains, with Labrador
and Greenland, many identical forms.
The shore waters from the southern peninsula of Alaska down to about Vancouver comprise the Aleutian province. Within this area occur some arctic species, notably the soft-shell clam, Mya arenaria, which the fishermen gather in such large quantities for bait along the Maine coast. An increasing number of species common to this west-coast faunal region and to the boreal province of the east coast are being recorded. Some of [pg312] these have been introduced from the East by the agency of man, but others no doubt have a natural range through the icy waters north of America and down through the Bering Sea into the Pacific.
The Californian province extends from the neighborhood of Vancouver to Cape St. Lucas and has about five hundred species. The northern portion of this area contains some very characteristic forms, together with a good mixture of species of more northern habitat; it might properly be regarded as a subprovince. From San Diego south to the cape the character of the fauna gradually changes; this coast-line of southern California has also been considered a subregion. The main characteristic of the Californian province is a large development of Haliotis, of the patelliform mollusks, and of the chitons. A very few minutes on any good collecting-beach of California would convince the Eastern conchologist that he had entered upon an entirely new field of research. The most notable genera are Calliostoma, Chlorostoma, Scurria, Acmæa, various genera of the Chitonidæ, Purpura, Monoceras, Amphissa, Norrisia, Olivella, Chorus, and Haliotis.
The warmer waters of the Gulf of California belong to the Panamic province, which extends down to South America. This is one of the richest and most interesting faunas in the world. At Panama alone, Mr. C. B. Adams, a noted collector and student of conchology, took upward of four hundred species. About eight hundred species are known in the Gulf of California alone.
It must be borne in mind that these faunal provinces relate to
littoral and shallow-water species, for the present knowledge of
deep-water forms is not sufficient to permit of generalization
upon their distribution.
We review here only the conspicuous forms along our own coasts. It may be well to note, however, that in some other parts of the world the development of molluscan life is far [pg313] in excess of anything to be found upon our shores. The veritable paradise of the naturalist is the East Indies. There the "aristocratic" genera, so called on account of their marvelous beauty, occur in their highest development—Voluta, Mitra, Oliva, Conus, and the various murices. There also are to be found the pearl-oysters, Meleagrina margaritifera, that yield their valuable harvest, and the giant clam, Tridacna gigas, which measures sometimes five feet in length. Over eight thousand species of mollusks are described from this surpassingly rich region, yet this vast province, as compared with the American and European shores, has been but superficially exploited by the naturalist. In whatever part of the world a naturalist may find himself, there is always a tempting array of molluscan life to attract him. Each fauna possesses features peculiar to itself, and from the point of view of the true naturalist, the more somber-hued and conventionally formed mollusks of Northern shores are no less interesting than the gorgeously tinted and fantastically shaped species of the tropics. From any faunal province of our own country one may readily gather all forms necessary to furnish ample material for study from which one may acquire an excellent idea of the biological features of the entire phylum.
The word "station" is used to indicate the nature of the surroundings which an animal chooses as most suited to his well-being. Some groups of mollusks, like the littorinas, the trochids, the purpuras, and the majority of those having patelliform shells, generally live on rocks above low-tide mark; other genera, like Buccinum, Sipho, and Chrysodomus, prefer rocky or gravelly ground below low-tide mark. Other mollusks burrow deep in the mud, many prefer sandy bottoms, while a host of other species seek homes upon the tangled masses of seaweed, living like arboreal creatures in the submarine forests. Nearly every conceivable character of sea-bottom or shore-line between tides harbors its own peculiar types of molluscan life. There are some very curious genera of bivalves that bore their way into the hardest rock and [pg314] there find a comfortable shelter for life. A numerous class of pelagic or free-swimming mollusks supposedly never go to the bottom at all, but spend the whole of their existence on or near the surface of the sea, always in open water, where their fragile shells may not be injured by rough contact with solid substances.
The food of some is vegetable, of others, animal. The bivalves, like the clams, oysters, cockles, and mussels, feed only on microscopic organisms. They create a current of water through their siphons, or mantle openings, and then, by a process best known to themselves, catch and swallow all the animalculæ thus brought to their mouths. The univalves which possess a siphon are, for the most part, carnivorous, and are often most voracious creatures. They feed upon any animal matter they can find, while some of them are enabled by means of a sharply toothed tongue to bore through the solid shells of other mollusks and extract the succulent vitals from within. One energetic little mollusk in particular, Urosalpinx cinerea, is for this reason a great pest upon the oyster-beds. Univalves not possessed of a siphon may generally be considered herbivorous; they pass most of their time peacefully browsing upon algæ.
Mollusks are all oviparous or ovoviviparous; that is, they lay eggs, or, laying eggs, they retain them within their shells until the young are hatched out. As a rule, each species of mollusk has its own particular method of protecting its eggs from external injury. Some construct tough, leathery capsules which are strung together in various patterns. The egg-capsules of Purpura lapillus, resembling little pinkish or yellowish club-shaped stalks, may frequently be found in the crevices of rocks and under the rockweeds. The egg-cases of Polynices (Lunatia) are most peculiar, resembling inverted gelatine-bowls with the bottom knocked out; when wet they are semi-elastic translucent masses in which may be seen myriads of eggs. Buccinum undatum arranges its hemispherical egg-capsules in layers one above the other. The number of eggs so deposited is often very great, running well into the thousands. Egg-capsules of Fulgur are leathery coils of angular disks adhering by one edge to a connecting-band of a similar texture. (See Plate I.) [pg315]
In bivalves the eggs are retained within the parent shell until hatched, and the young, which then escape into the water, are very different-looking creatures from the parent. Generally the young bivalve is free-swimming and exceedingly lively in its movements; sometimes by means of cilia it is enabled to attach itself to any passing host, and thus the species is widely disseminated. The fry soon lose their embryonic form and take on the characteristics of the adult.
The longevity of mollusks varies greatly with the species. Some attain an age of from ten to fifteen years, while some are only annuals. Oysters continue to grow for four or five years, after which they may survive many years more. In truth, little is known concerning the duration of life even of our better-known shore species.
In point of intelligence, mollusks must be relegated to a very low position. They give evidence of possessing no more than the most primary instincts, those of self-preservation and of reproduction. There are some forms that exercise a high degree of protective mimicry. The highest class of mollusks, the Cephalopoda (cuttlefishes, octopi, etc.), is widely separated from the other classes in the development of faculties that appear to correspond with intelligence. They are exceedingly crafty in the pursuit of their prey as well as in eluding capture.
Among the invertebrate animals, especially in the lower orders, wide physiological departures from central or typical forms are frequent. It often happens that a whole group or even suborder will differ so materially in its general characteristics from another group or suborder belonging to the same phylum that the student will be puzzled at first to understand how the two can be nearly related. In many such cases it is only when the essential features of the phylum are thoroughly understood that the relationships of its suborders can be appreciated. An example of widely divergent groups within the same phylum has already been shown in the Echinodermata. Who would at first suppose that the graceful Astrophyton could bear kinship to the inert [pg316] holothurian, or that both of these have the closest family ties with the sea-urchins?
The probable reason for these startling departures from a typical form is that in the lower orders of life, where the organs and their functions are comparatively simple, the laws of evolution operate far more quickly in the adaptation of an organism to environment and changed conditions of life. A decided tendency to this departure from the type is a feature of the mollusks. It would be difficult, if not impossible, to give a general description that would fit both a common garden-snail and the common oyster, yet both are mollusks. A concise definition of the phylum is therefore practically impossible, and it seems here as if nature resented all attempts to circumscribe her living forms by rigid rules.
The most persistent characteristic of the Mollusca is the possession of the "mantle," a sort of outer skin which, like a bag or sack, envelops the creature above and about the sides, but is open below, permitting the animal at will to protrude or withdraw its foot. From the outer surface of this sheath-like mantle, or fold, a calcareous shell is secreted. The modifications of this mantle are infinite; indeed, in some well-marked groups it seems to have wholly disappeared, having in reality become fused to the sides of the animal, and having become therefore a mere outer skin, and no longer a tunic or cloak, as originally intended. As already noted, the mantle does not always secrete a shell, a fact which again reminds us that we cannot give a good comprehensive description of the phylum that always holds true throughout its subdivisions. The best that can be done is to give in general terms the characters which appear in a majority of the forms. A convenient method of going about this is to create, for the sake of clearness, an ideal, model, or fanciful mollusk and clothe it with all the most salient molluscan features, and from such a basis to develop, as we go on, the various types of the actual orders and suborders. There is one class of mollusks, the Amphineura, which conforms reasonably well in most of its features to our comprehension of the schematic mollusk. This class is sometimes referred to by biologists as the "primitive" or "original" mollusks, from which all the other classes have since been derived; but the [pg317] evolutionary history of the development of the various molluscan orders through geological time does not seem to bear out the fact that an "ideal" mollusk, as we shall construct him, ever actually existed, or that his prototype, the Amphineura, was the first and consequently the ancestral type of the phylum. We must therefore only accept this ideal mollusk theoretically, to serve as a guide for the time being. We may profitably use it only as a lay figure—a sort of manikin, which will introduce us to the phylum. After our introduction we must forget our imaginary type and abandon any notion we may have conceived of the parental position of the Amphineura, its closest imitator in actual life.
The ideal mollusk has a compact, somewhat elongated body, the under or ventral portion of which is thickened into a muscular disk, upon which it creeps along. This disk is called the foot. (Note a snail thus crawling upon his "foot.") The foot is rounded or obtusely pointed posteriorly. Forward, the body is slightly narrowed into a neck, and then expanded again into a head. Upon this head are carried two projecting, mobile, fleshy tentacles, one upon each side, that superficially resemble two horns. At the base of each tentacle, upon the inner side, is an eye. Just below and between the tentacles is a small horizontal slit,—the mouth,—the lips being slightly thickened. The visceral portion of the animal is above the foot, or dorsal to it, and extends the whole length of the animal from the mouth to the excretory opening, the latter being situated posteriorly. The visceral cavity is inclosed above by a thin skin; indeed, it is but the great thickening of this skin below that constitutes the foot.