Zoölogy: The Science of Animal Life / Popular Science Library, Volume XII (of 16), P. F. Collier & Son Company, 1922

THE OYSTER AND ITS RELATIVES

The lowest in rank of these classes is the Pelecypoda, containing the "bivalves"—mussels, clams, oysters, and the like, in which the shell is in two parts or valves hinged together over the "back" of the animal, and attached to it on each side by a powerful muscle, the "adductor," by the contraction of which the shell may be tightly shut. Within the shell the body is enveloped in a "mantle," or fleshy membrane falling like a cloak on each side; and from it is secreted the outer shell, which grows by additions to its ventral margin. These additions are in a general way annual, so that the concentric lines of growth on its exterior are an indication of the years of the mollusk's life, which is slow in growth, and long-lived. The interior of the shell is usually pearly, and marked with microscopic rugosities, which, by breaking up the light, as if by innumerable prisms, gives the iridescence so beautiful in the pearl oyster, the fresh-water unios and many others. These pearly layers are called "nacre."

Bivalves were formerly classified in conchology as Acephala, because they have no proper head, but at the posterior end are two openings of tubes, provided with cilia. In one, the cilia induce a constant current of water which after leaving the gills brings into the animal's stomach floating microscopic food, both plants and animals, including eggs and larvæ, where it is captured and assimilated while water is ejected through the other (dorsal) pipe. This food includes bacteria, and if the mollusk lives and feeds in water polluted by sewage, or otherwise containing germs of disease, it becomes dangerous as human food; hence oysters and clams exposed to such bad conditions ought never to be sent to market because of the disease germs remaining in them.

In bivalves such as the oysters, horse mussels, piddocks, and others that are sedentary, and often fixed in place, or that, like river mussels, scallops, etc., move about freely, the mouth tubes are short; but many bivalves, as the clams, pinnas, razor fish and so forth, bury themselves in the sand of the bottom, by means of the strong distensible foot protruding from the forward end of the shell. These are provided with a double-barreled tube, called the "siphon," which may be contracted within the protection of the closed shell, or may be stretched out several inches; the animal may thus sink its body deep in the sand while its siphon reaches to the surface and inhales food-bearing water. The little squirts of water often seen jetting out of the beach at low tide as one walks along it are from clams so buried, and which, alarmed by the vibration of one's footsteps, hastily eject the water and withdraw their siphons.

The old name for this class, Lamellibranchiata, referred to the gills, two of which, on each side, hang like curtains inside the mantle and between it and the saclike body containing the viscera; when the shell is open they are laved by the water, and extract from it, by some quality hardly understood, the oxygen necessary to regenerate the blood that flows through them; and, in addition, respiration is carried on through the skin.

The nervous system is very primitive, and the sense organs consist of an otocyst (a minute sac in which a hard particle floats in a liquid) in the foot, by which, it is believed, a sense of direction is had, and which also serves the purpose of an ear; an organ that tests the water; and in some, as the scallop, rudiments of eyes are situated on the margin of the mantle. Most pelecypods are of two sexes, but some, such as our American oysters, are hermaphrodite. Eggs in vast number, and a cloud of spermatozoa, are thrown out in midsummer, and a little of the latter succeeds in reaching and so fertilizing fortunate eggs, but almost all merely serve as food for the host of mollusks, worms, sea anemones and what not that subsist on such provender. The few fertilized larvæ drift about and happily escaping multiplied perils, presently settle to the bottom to attach themselves to some fixed object, or otherwise get a chance to grow big enough to defy ordinary enemies. Some interesting variations in this rather commonplace larval history occur, however, in certain families.

It will be possible to name only a few of the most useful or otherwise conspicuous bivalves, beginning with the oyster, concerning which an immense amount of detailed information is accessible to the reader in the reports of the United States Government (Tenth Census, and documents issued by the Fisheries authorities) and in those of States, like Connecticut, New York, and Maryland, where oyster culture is an extensive industry, said to be worth in the aggregate about $20,000,000. The oyster of the eastern American coast is to be found in the Gulf of St. Lawrence, but not in considerable numbers between there and western Maine, whence it is present southward to the Gulf of Mexico, except on the shifting sands of the outer beaches. It seeks protected waters and a rocky or weedy bottom furnishing objects to which it may, when young, attach itself, and later will not be torn adrift by storms, for where an oyster establishes itself in infancy it means to stay all its life. Hence the sheltered waters of Buzzards and Narragansett Bays, Long Island Sound, and the lagoons and inlets that lie behind the outer line of sandy beaches from Long Island to Florida are the sources of our supply—especially Chesapeake Bay.

A full-grown oyster will produce about 9,000,000 eggs, each being about one five-hundredth of an inch in diameter. When the little oyster (spat) is about one-eighth inch wide shells begin to form on its sides, and it settles to the bottom with its left side down, usually where other oysters are; and hence extensive colonies, or "reefs," of these mollusks form, and "rise on their dead selves" to a level where they may be reached by the oysterman's rake. Many years ago, however, it was discovered that large, marketable oysters were becoming very scarce. Oystermen therefore sought favorable places, and raking the natural beds transplanted their catch, little and big, to new ground, where they were left to mature. This crude method was next improved on by sowing thickly over the new ground, just before spawning time in midsummer, a great quantity of empty oyster and other shells. These were favorable to the catching of "spat," and would result in a new bed that in about four years would furnish salable oysters; and annual plantings produced, after a time, an annual crop. These are the essential facts of oyster culture everywhere, although methods differ somewhat in other parts of the world—in France, for example, fascines of twigs are spread over tidal flats to catch the spat, instead of shells.

Our eastern American oysters are undoubtedly the largest and finest for the table of the many species that exist all round the globe. Those of the Pacific coast of the United States are excellent, but small; and the same is true of the European species; nor is the use of oysters abroad so general and extensive as in the United States.

The pearl-bearing oysters are somewhat distant relatives of the edible oyster (Ostræa), the thorny oysters (Spondylus), the hammer shell, the windowglass shell (Placuna) and others. The pearl oyster of commerce is named Meleagrina margaritifera and is found in scattered localities within the tropics on both continents. The chief fisheries are in the Persian Gulf, around Ceylon, in Australia, among the Sulu Islands and on the west coast of Panama. The Pearl Islands, south of Panama, yielded to the early Spanish adventurers riches in gems that rivaled those their competitors obtained from gold mines; but now they are a field of small importance. In fact, the pearl fishery is carried on now far less in hope of a profitable collection of gems than for the profit in the shells, which have a nacreous interior of remarkable beauty—the mother-of-pearl—and the great advantage of offering this in almost flat surfaces, sometimes eight or nine inches broad, making it useful in the arts as well as in the more practical line of buttons, knife handles, etc. Sometimes the whole surface of a fine shell has been carved, cameowise, with cunning art and an exquisite effect.

CHAPTER IX
BUILDERS OF THE PEARLY SHELLS—Continued

MUSSELS, SCALLOPS AND CHITONS

The familiar marine mussels of the family Mytilidæ will some day become of great importance in this country as a food supply, as now they are useful in resisting encroachment by the sea on certain parts of the coast. They exist in vast numbers on both our coasts, and elsewhere in the world, in two genera, Mytilus and Modiolus, which differ a little in form, but not in habits. They have acquired the stationary habit, and in place of a "foot" of serviceable size have developed a gland that secretes an exceedingly tough, fibrous bunch of threads known as a "byssus," by means of which the animal may not only attach itself firmly to any sort of object, but may actually move about. The common species of Modiolus, the "horse mussel," lives in great numbers north of Cape Hatteras at and below the line of low water, and is much larger than the edible mussel just described. A smaller species of Modiolus is extremely numerous on the New England coast, and down to the Carolinas, forming dense tangled beds on muddy patches as well as among rocks, and serving to bind the mud and plants together and hold them from disintegration by stormy waves, in spite of the thin and brittle character of their shells. A southern species is bright yellow, with dark rays; and the common modiola of the Pacific coast is dark, glossy brown. Such mussels are eaten regularly in Europe, and come to us in a pickled condition as a luxury. There is no reason why we should neglect to add our own to our long list of sea foods.

The next useful mollusk to be considered is the scallop, one of the many species of the family Pectinidæ, of which we eat only the adductor muscle. The commercial species is Pecten irradians, the name referring to the (nineteen) ridges that radiate from the flattened hinges to the scalloped margin of the shell, which is prettily colored. This species is common in sandy, shallow places from Cape Cod to Florida, but the fishery is most productive about the eastern end of Long Island and in Narragansett Bay. Farther north is a very much larger species (P. islandicus) especially abundant on the Grand Banks, off Newfoundland, where it forms an important food of the cod and other fishes. It is well known to cooks, who use it in baking their fish confections en coquille. A large number of other species are distributed throughout the world, one (P. jacobæus), inhabiting the Mediterranean having the name "pilgrim shell" in allusion to the fact that in the days of medieval religious pilgrimages, those who had visited the shrine of Saint James at Santiago de Compostela, Spain, to pay homage on July 25, were accustomed to wear a scallop shell in their hats in token of the fact—this mollusk being connected with traditions of that saint.

Turning to the fresh-water mussels, or naids, as some books call them, one is staggered to learn that more than 1,500 species have been named, a large proportion of which belong to the United States, which is peculiarly hospitable to them because of our many rivers and lakes, together with the prevalence of limestone rocks, whose constant dissolution in water supplies the store of calcareous matter that these thick-shelled mollusks require. All belong to the family Unionidæ, in which two divisions are noted—one (Anodon) in which the mussel has a comparatively elongated thin shell with no "teeth" in the hinges; and the other (Unio) in which the shell is thick, various in shape from an oval to a triangle, and has prominent umbones, beneath which the valves (which are always alike) are hinged together by interlocking teeth embedded in a somewhat elastic gristle. The interior of all these unios is richly nacreous, and consequently pearls are produced in the same way as in the marine pearl-bearing shells; and some of the finest known gems have been derived from them, in this country and abroad, as well as innumerable specimens of moderate value. These mollusks like clear streams or lakes with a sandy bottom, and are not to be looked for in stagnant weedy waters. They keep an erect position, the nibs of the shell half buried in the sand, and move slowly about, plowing a path and dragging themselves along by means of the powerful foot, but keeping the short siphons at the other (or longer) end of the shell well above the mud.

We come next to our market clams. These are of two distinct kinds—"hard" and "soft," or quahog and long clam, as they are distinctively called. The quahog is a thick-shelled, roundish mollusk with a distinctly heart-shaped outline when looked at endwise. It dwells in fairly deep water, standing on its nibs half buried in the sand, like a wedge, and moving slowly about. Young ones become the "little necks" of our summer tables.

The soft clam belongs to a different race. Its elongated shell is thin and chalky, is loosely hinged, and gapes widely at both ends, and although it is used much as food, especially in chowders, it is by no means as good as the hard clam. Its principal value, indeed, is as bait in the cod fisheries, and for this purpose enormous quantities are gathered. It lives in, rather than on, muddy beaches, sometimes in crowds of thousands, its shell deeply buried, and its long siphons reaching up to suck in water and food when the tide covers the flat. When the tide is out, a tiny hole in the sand and a spurt of water show the clammer where to dig, and his spade quickly unearths the clam.

The second class, Amphineura, contains the chitons and their relatives. These chitons are flattened mollusks protected by an armature of eight crosswise plates, overlapping like shingles, which creep about the rocks close to shore, and when lifted curl up like sowbugs. The most interesting thing about the chitons is the fact that they are provided with excellent visual organs, "the whole dorsal surface of some forms being studded with eyes, of which not less than 8,000 occasionally exist on a single specimen." Many of them are complete, with cornea, lens, and a pigment layer within the iris.

SNAILS AS TYPES OF GASTROPODS

The gastropods (Gastropoda), including the snails and slugs, limpets, whelks, periwinkles, sea hares and the like, are Mollusca having the mantle completely enveloping the body, and the shell, when present, in a single piece, and usually in spiral form. There is a well-developed ventral foot, on which the animal creeps, and in front of it a distinct head bearing eyes and tentacles. These organs retain their normal bilaterality, but the body is, as a rule, inequilateral. The cause of this is the fact that on the animal's back is developed from the first a shell, which, with its contents, amounts to a relatively large weight, and it naturally falls over to one side. The mouth is armed with a flat, distensible, ribbon-like organ, studded with rows of chitinous teeth, that serves as a rasp and a boring instrument, and which is called an odontophore, or, in snails, a radula. Most gastropods are carnivorous.

The lowest in rank are the shell-less, or "naked" gastropods known as "sea slugs," "sea hares," and so forth. One Mediterranean species of Aplysia secretes a purple liquid utilized by the ancients as a dye, and this is still sought for in Portugal, where storms sometimes cast vast quantities of the mollusk on the beaches.

We come now to the great group of mollusks inhabiting fresh waters and dry land—the snails, whose group name is "pulmonates," that is, possessors of lungs, and breathing air. On the generally accepted theory that all these are descended from marine ancestors, and have gradually acquired the faculty of living on land, it would be natural to look for a series of mollusks that were amphibious, and, as it were, half-way fitted for a terrestrial existence, and such intermediates exist in all parts of the world. The little black Melampus, which covers the mud of tide flats on both the Atlantic and Pacific coasts in tens of thousands, and seems just as happy when the tide is out as when it is in, or when it is simply refreshed by the spray, is a good example. A near relative, Carychium, is still more emancipated from the sea.

First among these pulmonates are those common in ponds and still streams the world over, of the family Limneidæ, called limneids or pond snails. They are in various forms. Some are limpet-shaped (Ancylus), some are flatly coiled (Planorbis), but most of them have shells drawn out into a graceful spiral; in all cases the shell is not composed of lime, but of the thin, fragile, horny substance "chitin." The best known one is Limnea stagnalis, which sometimes reaches a length of two inches, and inhabits almost every quiet piece of water in North America, and in Europe and all Asia except India and China.

These water snails of our ponds and ditches are exclusively vegetable feeders, and must come to the surface at frequent intervals to breathe, letting out a bubble of vitiated air, and taking in a fresh supply. Should the pond dry up in summer the limneids burrow down into the mud, and remain in that heat trance called æstivation until the autumnal rains refill the basin and let them come forth. The small kinds called "physas," exceedingly common everywhere in this country and Europe, differ from Limnea in having the shell partly enveloped in the turned-up fringed edges of the mantle, and by being coiled from right to left instead of clockwise. This reversal occasionally occurs in individuals of all gastropods, which are then said to be "sinistral," as opposed to the normal "dextral" coiling; but in the physas it is the rule.

Next come the wholly terrestrial pulmonates—snails and slugs, distinguished from the pond snails, which have only one pair of tentacles at the bases of which the eyes are embedded in the skin, by having two pairs of "horns," one of which carries the eyes on their tips—good eyes, which may be quickly withdrawn out of harm's way by inversion of the tubular stalks. The thick, extensible foot is surmounted by a body coiled within the shell; and this foot secretes a viscid fluid that lubricates the creature's path, and often leaves a silvery trail.

Snails are mainly vegetarians. The mouth lies just under the front tentacles, and its upper lip is armed with a horny, crescentic "jaw." Within the mouth is the lingual ribbon, which may be brought up against the cutting edge of the jaw. This tongue is studded with rows of infinitesimal, flinty teeth, the radula of our big white-lipped snail, a quarter of an inch long, furnishing room for 11,000 of these denticles; and as all of them point backward the tongue easily seizes and draws into the mouth whatever the jaw nips off. Substantially the same sort of "tongue" is possessed by all the gastropods, but the arrangement and shape of the microscopic denticles is different in every species, and this is one of the "characters" used in classification. With it the carnivorous rasp away their food; and by bending it double and using it as a gimlet bandits like Nassa, the oyster pest, drill through other shells and devour the occupant. You may pick up on any seabeach scores of examples of the work of these borers. In Europe some kinds of slugs and snails do great damage in gardens, but we have little to complain of in this respect.

Largely dependent on moisture, the young snails that are hatched in midsummer at once seek retreats, and may be looked for under leaves, logs, and loose stones in the woods and pastures. Most American snails are solitary, and will be found lurking in the moss beside mountain brooklets—a favorite spot for the glassy vitrinas—hiding in the crevices of rocky banks and old walls, crawling at the edge of swampy pools, creeping in and out of the crannies of bark on aged trees, or clinging to the underside of succulent leaves. Some forms, very beautiful in their ornamentation when magnified, are so minute that they might be encircled by the letter o in this type, yet you will soon come to perceive them amid the grains of mud adhering to the undersurface of a soaked chip or rotten log.

For fresh-water species various resorts are to be searched. Go to the torrents with rocky bottoms for the paludinas and periwinkles (Melania); to quiet brooks for physas and coil shells (Planorbis); for limneas to the reeking swamps and weedy ponds. By pulling up the weeds gently, you may get small species that otherwise easily escape your dipper or net. In the Southern States and in the tropics certain forms are to be picked off bushes and mangrove trees like fruit, especially the round "apple snails" (Ampullaria) as big as your fist.

SEA SHELLS IN NATURE AND ART

Other familiar forms of gastropods are the limpets, keyhole and half-deck; the abalones, so much used in the making of ornaments; and the many small sorts of "periwinkles" studding the rocks and hiding among the seaweeds of every coast. Then there are the pyramidal top shells (Trochus), the bulging, wide-mouthed turbans (Turbo), and the open-whorled wentletraps (Scalaria) which years ago were so rare that collectors paid $100 or more for a good specimen. The two former kinds are on sale in all seaside shops, with the natural rough brown exterior ground away until they gleam outside in the prismatic glory of the nacre layers that lie underneath. A group of heavy shells of carnivorous tropical mollusks furnishes ornaments for the mantelshelf also. These include the knobby volutes, often richly colored in marbled patterns or in spiral rows of round spots; the olives, whose ovate shells are sometimes dark purple, sometimes beautifully marked, and always glossy, because enfolded during life inside flaps of the mantle that completely protect them; the miters, that take their name from their resemblance in shape to the headdress of a bishop, and show splendid decorations in tints of red and orange; and the strong, spiny murexes, a small Mediterranean species, which is the principal source from which the ancients derived their Tyrian purple dye—a coloring matter yielded by treatment of the blood of many species, including one of the commonest little mollusks (Purpura) on our own coast, which old-fashioned New Englanders yet utilize sometimes for making an indelible ink for marking clothing. To this family belong the "drills" that destroy thousands of dollars worth of oysters annually in Long Island Sound by boring through them. Near relatives are the whelks (Buccinum), extensively eaten in England; and two of the largest and commonest shells on our eastern sand beaches, known to northern fishermen as "winkles" and along the southern coast as "conchs." These (Fulgur and Sycotyphus) are big, pear-shaped creatures with chalky white shells that crawl about near shore, seizing and devouring anything they can overcome, and working havoc on planted oyster beds; they deposit their eggs in parchmentlike capsules shaped like gun wads and connected into a long chain that are often thrown up on the beach, where they are called sea necklaces.

Of great beauty in their rich variety of color and pattern are the tropical cone shells, of which a large number of species are known, some so rare as to bring great prices in the conchological market. Their bite is poisonous. Equally numerous in species are the charmingly decorated auger shells, some (Pleurotoma) spindle-shaped, others (Terebra) that would serve as models for a church spire. Near them is classified that white mollusk (Natica) whose globular shell is perhaps the commonest relic of the sea seen on our northern beaches, and sometimes is as large as a man's fist; to it belong the curious "sand saucers" to be found in August, which contain its eggs. These naticas are predatory, and burrowing their way through the loose sand come upon and devour other shellfish, boring a circular, nicely countersunk hole through their armor and feeding on its inmate; their depredations on the northern oyster beds are a serious matter.

Well known and always admired are the cowries, smooth, brightly colored shells, shaped like an olive with a gash down the length of one side. This long and narrow aperture is usually toothed, and it is only in the young that any indication of a typical spiral growth is discernible. The money cowrie of Africa is small and cream-white.

Lastly a word must be said about the largest of known gastropods, the big "conchs" or wing shells (Strombus), the helmet shells (Cassis), and the tuns (Dolium). They are West Indian. The species most commonly seen in the United States, forming a border for flower beds in seaside villages, is Strombus gigas, with a delicate orange-red or pink interior, from which are cut most of the shell cameos offered to art lovers. This shell, like the great spiral triton of the South Seas, is also converted into a horn much used in foggy weather by the spongers and small coasters of Floridian and West Indian waters. The helmet shell, a heavy, rounder and smoother mollusk than the Strombus, is also extensively used in cameo cutting, especially the African black helmet, in which a white outer layer covers an almost black underlayer on the broad lip. Dolium has a large, globose but thin shell, ornamented with revolving ribs.

The class Scaphopoda is composed of a single family (Dentalidæ) known as tusk shells, because the little shells, one to two inches long, are shaped like an elephant's tusk, open at both ends. The structure of the occupant is so singular, the animal lacking head, heart, gills, and some other ordinary features, that naturalists believe it is a hopeless degenerate. One of the species of the Pacific coast is famous as the shell strung as ornaments and serving practically as money among the northwestern Indians until very recent times, under the name "hiqua."

NAUTILUS, DEVILFISH, AND SQUID

We have now arrived at the last and highest division of the Mollusca—the Cephalopoda, the class of the nautilus, ammonite, and other fossil forms, and of the squid, cuttles, and octopuses of our modern seas. The cephalopods are very different in shape, activity, and in their higher organization and intelligence, from other mollusks, but their general anatomy is the same. The special characteristic, as indicated by the name, is the fact that the head is surrounded by tentaclelike extensions of the "foot," which is here fused in part with the head, and divided into the long "foot arms," which are the instruments by which these predatory creatures obtain their prey. The underpart of the foot forms a tube called the funnel (or siphon). Through the funnel the animal expels water from the mantle cavity, and thus propels itself through the water. When the siphon is in its normal position the animal swims backward; but it can be turned back over the edge of the mantle, giving a forward movement. In cephalopods the sexes are separate, the male being often much smaller than the female. The eggs are usually laid in gelatinous capsules, commonly known in New England as "sea grapes," and the development is direct, that is, without any free-swimming larval stage.

The class is divided into two subclasses: 1. Tetrabranchiata, cephalopods with four plumelike gills inside the mantle; and 2. Dibranchiata, with only two such gills. In the first subclass belong all those very ancient cephalopods called in a general way ammonites, goniatites, orthoceratites, etc., that are found in such great numbers and astonishing variety in the Paleozoic rocks, from the Ordovician age onward, although but few groups survived beyond the Carboniferous period, and only two families can be traced as high as the Tertiary deposits, one of which—that of the nautilus—survives to the present day as the final remnant of one of the conspicuous and interesting populations of the primitive ocean.

The pearly or chambered nautilus is one of several species inhabiting the East Indies and the coral region of the South Pacific seas, creeping along the bottom in deep water, most numerously at the depth of about 1,000 feet. Hence the animal is not often taken alive, although the smoothly coiled and handsome shells are cast on the beaches in great numbers; and little is known of its habits or embryology. It is a soft lumpish sort of creature, with a great number of short arms and tentacles around the mouth, none armed with suckers. It begins life as a mere globule covered by a minute hood of shell; but presently, growing too large for this hood, it enlarges it by additions to the rim, and then forms behind its body a partition (septum) across the shell, cutting off the part in which it was born. As growth advances, this enlarging and partitioning continues until the nautilus has attained its full size. Then, as before, it occupies only the outermost chamber, behind which the whole interior of the shell is divided by the septa into chambers, abandoned and empty, but filled with a gas that buoys it up in the water. Oriental artists are fond of grinding away the dull exterior of the shell and exposing the gleaming nacre underneath; and of carving in this mother-of-pearl picturesque designs, examples of which are often to be seen in curiosity shops. This is not only the last remnant of the great group of ancient nautiloids, but one of the smallest, for some of the Paleozoic coiled forms were as big as a washtub, and the straight ones were often six feet long.

The Dibranchiata, on the other hand, are comparatively modern, as their ancestry dates back only to the Trias, and our seas still harbor a long list of living representatives. This subclass has two divisions: 1. Octopoda—octopods, the eightarmed argonaut and other octopuses; and 2. Decapoda—decapods, the ten-armed cuttlefishes, or calamaries, and the squids.

The octopods have a saclike body with eight arms of about equal size, in some kinds thick and short, in others long and snaky. Every arm has along its underside a double row of round, muscular suckers without horny rims; and whatever is seized by one or more of these arms is drawn into the mouth at their base, where it is bitten by a beaklike jaw of sharp horn, and further devoured by means of a toothed tongue similar to the radula of gastropods. Nearly all are tropical, but some species exist in deep water considerably to the northward. Certain species are used as food in many parts of the world, and are considered a delicacy in Italy and other Mediterranean countries. The fishermen of Japan and the Philippines capture them by the simple process of lowering big earthen urns and leaving them on the bottom overnight; when they are hauled up in the morning many will contain entrapped devilfish, as sailors call them, which at once go to market.

A very singular octopod is the little argonaut, or "paper sailor." Its body is not larger than a walnut—that is the body of the female, for the male is only a tenth of that bigness. Its home is mainly in the tropics and in deep water, but in the summer spawning season it rises to the surface, and is occasionally met with far northward on the Gulf Stream, drifting, apparently, in a snug little boat. The two dorsal arms are expanded into broad, roundish membranes at their ends, and old stories said that they were used as sails—a supposition of much use to poets; but the "boat," shaped somewhat like the shell of the nautilus, is not a shell proper, but a membranous pouch secreted by the mantle in spawning time, and not vitally attached to the body, but held in place beneath it by the two broadened arms, and serving as a receptacle for eggs and a cradle for the embryos hatching from them.

Turning now to the Decapoda, we treat of things much nearer home and familiar on both sides of the continent, for these are the cuttlefish and squids, none of which have an external shell, but possess an interior brace to their muscles either of lime or of chitin. The cuttlefish proper, or calamaries, are those of the family Sepiadæ, which have an oval, flattened body bordered by a fin; and two of the ten arms are, in the female, in the form of long, slender tentacles. In addition to being edible and easy to get, as they stay near shore, their calcareous back brace is the "cuttlebone" fed to cage birds; and they furnish the substance from which the drawing ink called "sepia" is made—principally in Rome. This is a brownish black liquid that the animal jets out through its siphon when it thinks itself in danger in order to make an inky cloud in the water behind which, as a sort of smoke screen, it may run and hide. Other cephalopods use this means of escape.

The squids, however, are all elongated in shape, and have finlike expansions of the mantle only on the tail. Two of their arms are long and slender, and are broadened at the tips, and studded with suckers. These suckers in some squids are strengthened by a horny rim, or by recurved hooks, or by both. The eyes are large, perfectly formed, and as serviceable as those of the fishes on which they prey. These, and some other animals, including small ones of their own kind, they capture by darting backward, swinging quickly to one side and seizing the victim in their sucker-bearing arms. They themselves are devoured by whales, seals, and many kinds of fishes; and enormous quantities of squids of various species are annually collected by fishermen for use as bait in the Newfoundland fisheries. In place of the calcareous cuttlebone of the sepia the squids have their bodies stiffened by an internal strip of chitinous substance called the "pen."

Squids are of all sizes from an inch to twelve feet in length; then there is a surprising jump to the giants (Architeuthis) of the North Atlantic, which, when the tentacles are stretched out in front, may measure seventy-five feet from tip to tail. These are little different in structure or habits from their smaller brethren that exist in so many species near all coasts and throughout the midseas right around the globe; but their huge size makes them fit antagonists of the sperm whale, which hunts them, and whose hide often bears a record, left by their powerful suckers, to show how hardly some big squid struggled for life. These monsters are the greatest invertebrates known in present or past time; and it is probable that the long wriggling arms of one and another, glimpsed at the surface, may account for some of the sea serpent stories brought home by apparently perfectly honest sailors, especially those which in many cases recount that the supposed "serpent" was in conflict with a whale. Carcasses of these gigantic squids are occasionally cast on the shores of Labrador and Greenland.

CHAPTER X
ANIMALS WITH JOINTED FRAMES

The phylum Arthropoda embraces an immense assemblage of small animals, inhabiting salt and fresh waters, the land, and the air above it. The typical members of this group have a body divided into segments, jointed limbs, some of which are modified into jaws, and a more or less firm external skeleton. The general organization is complex, with the nervous system and senses well developed, in some divisions showing powers of perception and brainwork of a very high order. The chief divisions, or classes, of the Arthropoda are given below in the order of rank, from those simplest in organization to the most complex. Members of the first three classes breathe by gills, and are termed Branchiata, the remainder are air breathers or Tracheata.

As several of these classes contain many subdivisions, and thousands or even tens of thousands of species, all that is possible is to give the reader such an account of each important group, as will enable him to assign to their proper place such arthropods as he may encounter in his rambles, or in his reading, and to learn something of the manner of life in the various groups.

CRABS AND THEIR SMALL RELATIVES

"Everyone," says Dr. Calman, "has some acquaintance with the animals that are grouped by naturalists under the name Crustacea. The edible crabs, lobsters, prawns, and shrimps are at least superficially familiar, either as brought to the table, or as displayed in the fishmonger's.... Many, however, will be surprised that the barnacles coating the rocks on the seashore, the sand hoppers of the beach, and the wood lice of our gardens, are members of the same class. Still less is it suspected that the living species of the group number many thousands, presenting strange diversities of structure and habit, and playing an important part in the general economy of nature."

The great majority of crustaceans are aquatic animals, breathing by gills or by the general surface of the body, having two pairs of "feelers," or antennæ, on the front part of the head, and at least three pairs of jaws. Most crustaceans are hatched from eggs, usually in a form very different from their parents; and they reach the adult state only after passing through a series of transformations quite as remarkable as those that a caterpillar undergoes in becoming a butterfly. All crustaceans, except a few much modified land forms, breathe by means of feathery or platelike gills which are always an appendage of the legs, where they appear as one or more lobes. Colorless blood propelled by the heart wanders into spaces in these lobes, and there lies separated from the water by a mere film of tissue, through which oxygen is absorbed from the water. Most crustaceans are covered, at least in part, by some sort of shelly coat composed of a combination of the horny substance "chitin" with lime, which reaches its highest state in the big lobsters and crabs. This not only protects and gives support to the internal organs, but also to the muscles by which the animal moves. In other words it plays the part of a skeleton. As it does not increase in size after it is once formed, and cannot stretch much, the crab must cast its shell at intervals as it grows. The new covering, which had been formed underneath the old, before molting, is at first quite soft, and the animal rapidly increases in size owing to the absorption of water. The shell then gradually hardens by the deposition of lime salt.

The reader who may not hitherto have understood the difference between "hard" and "soft-shelled" crabs is now instructed; and it is observable that the figurative expression "a hard-shell," when applied to a man, signifies that he must undergo a complete change before his ideas will be enlarged.

The simplest of the crustaceans are those small creatures of the subclass Branchiopoda (gill-footed) that swarm in our waters, both salt and fresh. Lakes, ponds and ditches abound in a variety of minute or even microscopic species that, in gathering food from equally small bits of dead organic matter, as well as from living plants and animalcules, perform an important service as scavengers—a service, in fact, performed by all crustaceans in a greater degree than by any other single group of animals. They also furnish the basis of food for the whole body of aquatic life, since it is upon these minute crustaceans that fish fry, tadpoles, insect larvæ, caddis flies, and so on, must mainly depend. One of them is Daphnia, familiar to keepers of aquariums. Another is Cyclops, a favorite with microscopists and abundant in stagnant ponds, which is a member of the group called copepods that form an important part of the oceanic plankton, where they are the chief consumers of the minute algæ; but they also occur at all depths. In arctic waters the copepods are so abundant that they form the principal part of the food of certain fishes and of the whalebone whales. These, and their minute relatives, the ostracods, produce a large part of the phosphorescence of the sea, and some of them exhibit bright colors.

All these are free swimmers, but nearly related to them are the barnacles (Cirripedia) whose larvæ float about for a time near shore, and then settle down and attach themselves by their hinder parts to a rock or some other support, and begin to secrete an armature of limy overlapping plates that forms a strong cup in which they sit, often in a crowd that whitens a big rock. When the tide is low these sessile "acorn shells" are tightly closed, but when the water returns, bringing its load of invisible food, the animal stands up, as it were, and thrusting out its feathery legs sweeps the water to capture a meal—a beautiful sight to watch. The relation of the plates in the barnacle's cup to those in the coat of the higher Crustacea is more easily seen in the more pelagic "goose barnacle," whose hinder part is extended into a tough, flexible stalk, while the fore part is covered by plates. This kind is fond of attaching itself to floating timber, to ships' bottoms, or even to the surface of whales, and thus floats or is carried all over the watery globe. To it belongs the ridiculous myth of the barnacle geese.

Great numbers of crustaceans of more advanced types live in the open sea, and at all depths; and many of them assume extraordinary shapes. The space between tide marks, and the mud of salt marshes and tidal creeks abound in a wide variety of species, some of which are familiar to everyone who lives at or visits the seashore. Thus the sand and rows of drifted seaweed on all our eastern beaches are likely to harbor flocks of amphipods, well called "sea fleas" or "sand hoppers," which sometimes jump away before you in hundreds as you walk along.

Here, too, are to be found the pretty, burrowing "mole crabs," or "ivory crabs," so called from their shining white jackets; and a host of other species with strange forms and habits haunt the margins of tropical and Oriental seas. All these are bandits, preying on whatever they can catch, and between times guarding themselves from capture by fishes, bigger crabs, and other enemies, by lying in mud burrows, to the bottom of which they are quick to retreat. The big arm of the fiddler crab, held across its face, closes its burrow like a door. One sort, the hermit crab, has all its hinder parts naked, and so backs into an empty snail shell, curling its taillike soft abdomen around the central column of the shell and so dragging it about with it, with its armored head and thorax sticking out of the mouth of the shell. As it grows it becomes too large for its first shell, and from time to time must leave it and find a larger tenement in which to ensconce itself—a perilous transfer. Let me quote some notes I made on a New England shore to give a picture of crustacean life there in summer.

"The lady crabs were plentiful, always alert, and inclined to be pugnacious at our intrusion. The first one I met instantly rose upright at the surface of the water, and when I made an advance it sprang half way out of the water and cracked its pincer claws together as if supposing it would reach, or at any rate frighten me. Perhaps it was my shadow it clutched at so viciously. If so, the crab probably concluded its huge antagonist to be an intangible ghost upon which the most powerful claws could have no effect, for an instant later it backed down—literally and swiftly—to the bottom, and in a twinkling had wriggled tailwise into the mud and out of sight. When with my shovel I routed madame out of that retreat, she indignantly scuttled off too briskly to be followed, and will have great tales to tell of her adventure.

"The stone and fiddler crabs were as common and comical as usual; and I made the acquaintance of a new one called Gebia, which was a small, semi-transparent, bluish white, washed-out, bloodless specimen, shaped somewhat like a crawfish and carrying bunches of roe beneath its abdomen. It looked like a miniature lobster made of glass and filled with milk. Then in the eelgrass there was a funny isopod, called Caprella. It was half an inch or so long, and clung by its hinder feet to the grass, waving its body up and down in search of minute prey. Other isopods and amphipods were exposed by turning over stones or digging in the sand at the edge of the water—small, pale, shapeless crustacea, which are flattened laterally so that they must lie on their sides, and when uncovered will kick about with feet and tail in laughable anxiety to get under something. Under the stones we found the tubes made by a certain species; and when we captured the active little architect and put him in a bucket of clean water, he instantly began to gather grains of sand and stone and to join them together Into a shield under which he might hide. We found that these grains were joined together by spiderlike threads, which the amphipod spins from two pairs of small legs under the middle of his body, secreting a fluid that hardens in the water. Another (Hippa) about the size and shape of a robin's egg, but with a thin shell of mother-of-pearl (so to speak), gave us great amusement by its extraordinary celerity in burrowing, so that we could hardly seize it before it had squirmed down out of reach into the wet sand."

The edible crabs (Cancer) live in the shallow region just below ebb tide, for they cannot endure exposure to air as well as other species, and live by scavenging. The lobsters are inhabitants of still deeper water, especially where it is somewhat rocky, and devour more carrion than living fish. That miniature of the lobster, the fresh-water crawfish, which is also edible, dwells in deep burrows in wet lands—burrows that are really wells half filled with water. Various species of these and other edible forms of Crustacea are found all over the world.

MILLIPEDES AND CENTIPEDES

The myriapods (class Myriapoda) are those unpleasant creatures more commonly known as centipedes, millipedes, or thousand-legged worms. They have a wormlike form, with the body divided into segments, a distinct head with antennæ, jaws and several single eyes, and a varying number of air tubes, or tracheæ; two sexes exist, and eggs are laid in the ground within cases formed by the mother of pellets of mud. They vary in size from an almost invisible minuteness to a length in some tropical species of six or more inches. The centipedes (Chilopoda) are those flattened forms so often seen in and about rotting wood and vegetation or in moist ground, their bodies looking like a chain of plates joined together by flexible skin, each section having a single pair of legs, usually very short, but in one sort (Cermatia) each leg is longer than the body, and the hinder pair twice as long, matched by two very long feelers. Most of them are predacious, feeding on anything they can catch, and their strong jaws exude poison. The larger ones may inflict a very painful bite if incautiously handled.