Note that the mouth leads by a short tube (_sophagus_ or _gullet_) into a large cavity, the _stomach_, which is surrounded by a greenish ma.s.s, the _digestive gland_.

TECHNICAL NOTE.--Carefully cut the delicate covering of the dorsal portion of the visceral ma.s.s and expose a cavity.

The cavity thus exposed is the _pericardium_. Note within the pericardium a long tube extending through it. This is a portion of the alimentary ca.n.a.l, the _r.e.c.t.u.m_, which opens posteriorly through the _a.n.u.s_ into the _supra-branchial chamber_. Note a muscular sac about the r.e.c.t.u.m midway of its course through the pericardium. This is the unpaired _ventricle_ of the _heart_. Attached to each side of the ventricle are thin-walled sacs, the _right_ and _left auricles_, which are entered by fine blood-vessels, the _efferent branchial veins_, from the right and left gills. The blood brought through these blood-vessels from the gills flows into the auricles and from them into the unpaired muscular ventricle, from which it is forced anteriorly and posteriorly through two main arteries, the _anterior_ and _posterior aortas_, to all parts of the body. After bathing the body-tissues the blood is collected into a median longitudinal vein beneath the pericardium called the _vena cava_. From the vena cava the blood pa.s.ses through the kidneys and gills to be returned at last to the heart. The mantle acts as an organ for the aeration of the blood, and the blood it receives or at least part of it pa.s.ses directly back to the heart without pa.s.sing through the kidneys and gills.

Note the delicate membranous dark-colored sac on the floor of the pericardium, the _kidneys_ or _nephridia_. These are paired structures which appear as two U-shaped tubes lying side by side. Each consists of a lower portion with thick folded walls, the kidney proper, and an upper thin-walled portion, the _ureter_. The kidneys open internally through a pair of _reno-pericardial_ openings into the pericardium, while the ureters communicate with the mantle-cavity by an opening on the side of the body beneath the gills as already mentioned. The kidneys are profusely supplied with fine blood-vessels and carry off the waste matter from the blood.

Beneath the posterior adductor muscles note a small white spider-shaped body, the more or less united _visceral ganglia_ of the nervous system. Posteriorly these ganglia give off nerves to the mantle and gills, while anteriorly there proceed two nerves, the _cerebro-visceral connectives_, running forward, one on either side of the foot close to the visceral ma.s.s, to the _cerebro-pleural ganglia_, paired ganglia lying near the mouth. A delicate commissure running over the gullet connects these ganglia.

TECHNICAL NOTE.--Cut away the skin and outer muscular layer from the left side of the foot.

Note the large stomach-cavity, surrounded by the digestive gland. Trace the convolutions of the alimentary ca.n.a.l through the foot to the a.n.a.l exit. Note in the anterior portion of the foot a fused pair of ganglia similar to the visceral ganglia. These are the _pedal ganglia_, which are connected by a pair of delicate commissures, the _cerebro-pedal connectives_, with the cerebro-pleural ganglia. Note the glandular tissue which fills the cavity of the foot and surrounds the loops of the alimentary ca.n.a.l. This is the reproductive organ, which has its exit beneath the gills on each side of the foot. The s.e.xes of the mussel are separate, but the reproductive organs are very similar.

=Life-history and habits.=--The eggs (ova) of the female pa.s.s first into the supra-branchial chamber, whence, after being fertilized, they drop into the outer pair of gill-chambers. These outer gills serve as brood-pouches, and here it is that the embryonic stages are pa.s.sed through. The embryo when ready to issue has a soft body enclosed in two triangular valves. At this stage it is called a _glochidium_. The glochidium on being discharged through the exhalant siphon of the parent falls to the bottom, where it remains for a time, when it attaches itself to some fish by the lower hook-like projections of the valves and leads a truly parasitic life for two months, after which it undergoes a metamorphosis and falls to the bottom again, there to begin an independent existence. Mussels often congregate in favorite mud or sand banks. Their food consists primarily of small organisms, both plants and animals, which are taken from the water entering the mantle-cavity. Mussels move about slowly over the muddy bottom of the stream by means of the muscular foot.

OTHER MOLLUSCS.

The branch Mollusca includes the fresh-water mussels, the clams, oysters, snails, and slugs, the cuttlefishes, and all that host of animals we call "sh.e.l.ls" or sh.e.l.l-fish, which we know familiarly only by the sh.e.l.l which they make, live in, and leave at death to tell the tale of their existence. Not all the molluscs, however, form sh.e.l.ls, that is, external sh.e.l.ls which serve as houses. The familiar slugs do not, nor do a number of ocean forms called nudibranchs, which are somewhat like the land-slugs, only much prettier and more attractive.

All the cuttlefishes and octopi are also without the hard calcareous sh.e.l.l. But most of the molluscs are sh.e.l.l-bearing animals. The sh.e.l.l may be bivalved, as in the mussel and clam, or univalved, that is, composed of a single piece which may be spirally twisted, as with the snail, or otherwise curiously shaped. The variety in the form, colors, and markings of the sh.e.l.ls indicates the great diversity among molluscs. Molluscs live on land, in fresh water and in the ocean. No depths of the ocean abysses are too great for the octopi, no coast but has its many sh.e.l.ls, hardly a pond or stream is without its mussels and pond-snails, and in all regions the land-snails and slugs abound.

=Body form and structure.=--The molluscs are not to be mistaken for any other of the lower animals; they have a structure peculiarly their own. In them the body is not articulated or segmented as with the worms and arthropods, nor radiate as in the echinoderms, nor plant-like as with the sponges and polyps. (Where the typical molluscan body is well developed it is composed of four princ.i.p.al parts: a head, with the mouth, feelers, eyes, and other organs of special sense; a trunk containing the internal organs; a foot which is a thick muscular ma.s.s not at all foot- or leg-like in shape, but which is the organ of locomotion by means of which the mollusc crawls; and a mantle which is a fold of the skin enclosing most of the body and which produces the sh.e.l.l. Such a typical molluscan body is possessed by most of the snails. But in most of the other molluscs one or more of these four body-regions are so fused with some other region as to be indistinguishable. In the mussels and clams the head is not at all set off from the rest of the body, the cuttlefishes and octopi have no foot, the slugs have no sh.e.l.l. In the case of some of the molluscs without external sh.e.l.l there are inside the body the rudiments or vestiges of a sh.e.l.l.

With regard to the internal organs we note the constant presence of three pairs of ganglia, viz., the brain, lying above the pharynx, which sends nerves to the feelers, eyes, and auditory organs; the pedal ganglion, which sends nerves to the foot, and the visceral ganglion, which sends nerves to the viscera. This is a condition of the nervous system characteristic of all molluscs. The heart is a well-developed pulsating sac in the upper part of the body composed of either two or three chambers, and there is a well-defined closed system of arteries and veins, specially complete in the cuttlefishes and octopi. This highly developed condition of the circulatory system also distinguishes the molluscs from the other invertebrates.

=Development.=--Reproduction among the molluscs is always s.e.xual.

Multiplication by budding or by the parthenogenetic production of eggs is not known to occur. The eggs are usually laid in a ma.s.s held together by a gelatinous substance. In most species the young mollusc on hatching from the egg does not resemble its parent, but is a free-swimming larva called a _veliger_. It is provided with cilia for organs of locomotion.

It must undergo a radical change in order to reach the adult stage. Thus metamorphosis occurs in this branch as well as among the Arthropods and Echinoderms. In the development of some molluscs, however, there is little or no metamorphosis, the young being hatched in a condition much resembling, except in size, the parent.

Some of the special characteristics of structure, life-history, and habits of the molluscs will be noted in our consideration of the various kinds.

=Cla.s.sification.=--The branch Mollusca is divided into five cla.s.ses, three of which include the more familiar kinds. These three cla.s.ses are the Pelecypoda, including the mussels, c.o.c.kles, clams, scallops, oysters, etc., molluscs with a sh.e.l.l composed of two pieces, one on each side of the body and hinged together; the Gastropoda, including the snails, slugs, periwinkles, whelks, and a host of other univalved sh.e.l.l-fish, that is, molluscs which have a sh.e.l.l composed of a single piece; and the Cephalopoda, including the squids, cuttlefishes, octopi, and the pearly nautilus.

=Clams, scallops, and oysters (Pelecypoda).=--TECHNICAL NOTE.--Sh.e.l.ls of scallops, oysters, and sea-mussels should be had for examination; also specimens of _Teredo_ or _Pholas_ in alcohol or formalin, and pieces of pile bored by _Teredo_. Make drawings of various bivalve sh.e.l.ls, and of _Teredo_.

[Ill.u.s.tration: FIG. 103.--Dissection of fresh-water mussel, _Unio_ sp.]

The fresh-water mussel which we have studied is an example of the bivalve molluscs. The members of this cla.s.s show a range in size from the little fresh-water _Cyclas_ about 1 cm. long to the giant clam of the Indian and Pacific islands "which is sometimes 60 cm. (2 feet) in length and 500 pounds in weight." They show also some variety in the form and appearance of the sh.e.l.l, but not anything like the degree of variety shown by the sh.e.l.ls of the Gastropods.

The edible clams are of several different species. The hard-sh.e.l.l clam (_Venus mercenaria_), or "quohog" as it is often called, is found along the Atlantic coast from Texas to Cape Cod. It is "common on sandy sh.o.r.es, living chiefly on the sandy and muddy plots, just beyond low-water mark.... It also inhabits estuaries, where it most abounds.

It burrows a short distance below the surface, but is frequently found crawling at the surface with the sh.e.l.l partly exposed." The sh.e.l.ls of this edible clam are white. The soft-sh.e.l.l clam (_Mya arenaria_), "the clam _par excellence_, which figures so largely in the celebrated New England clam-bake, is found in all the northern seas of the world....

All along the coasts of the eastern States, every sandy sh.o.r.e, every mud flat, is full of them, and from every village and hamlet the clam-digger goes forth at low tide to dig these esculent bivalves. The clams live in deep burrows in the firm mud or sand, the sh.e.l.ls sometimes being a foot or fifteen inches beneath the surface. When the flats are covered with water his clamship extends his long siphons up through the burrow to the surface of the sand, and through one of these tubes the water and its myriads of animalcules is drawn down into the sh.e.l.l, furnishing the gills with oxygen and the mouth with food, and then the water charged with carbonic acid and faecal refuse is forced out of the other siphon. When the tide ebbs the siphons are closed and partly withdrawn." Ocean clams and mussels have furnished food for man for ages, and along coasts are found here and there great mounds made of heaps of clam-sh.e.l.ls which have become covered over with soil and vegetation. Such mounds are the old feasting-places of the early coast inhabitants, and the archaeologist often finds in these "kitchen-middens," as they are called, various relics of the early natives of the continent.

[Ill.u.s.tration: FIG. 104.--A group of marine Pacific Coast molluscs; in upper left-hand corner, _Purpura saxicola_; next to the right, _Littorina scutulata_; farthest to right, limpets, _Acmara spectrum_; left-hand lower corner, _Mytilus california.n.u.s_; in right-hand lower corner the black sh.e.l.ls just above the large clam-sh.e.l.l, _Chlorostomum funebrale_. (From living specimens in a tide pool in the Bay of Monterey, California.)]

Even more widely known that the clams are the oysters (_Ostrea virginiana_), also members of this cla.s.s of molluscs. The oyster is carefully cultivated by man in many countries. It has its two sh.e.l.ls or two sh.e.l.l-halves dissimilar, one valve being hollowed out to receive the body, while the other is nearly flat. The oyster is attached to the sea-bottom by the outside of the hollowed-out valve.

When first hatched the young oyster swims freely by means of its cilia; after a few days it attaches itself to some solid object and grows truly oyster-like. Much care has to be taken in cultivating oysters to furnish proper conditions for growth and development. The young oysters when first attached are called "spat"; when a little older this "spat," now called "seed," may be transplanted to new beds, which are stocked in this way. In fact some beds have constantly to be thus restocked, the young oysters produced on them not finding good places to attach themselves, and so swimming away. Sometimes pieces of slate, pottery, etc., are strewed about the oyster-beds to serve as "collectors," that is, as places for the attachment of the young oysters. The extent of the acreage of the American oyster-beds is larger than that of any other country. "The Baltimore oyster-beds on the Chesapeake River and its tributaries cover 3,000 acres, and produce an annual crop of 25,000,000 bushels."

[Ill.u.s.tration: FIG. 105.--_Dactylus_ sp., a mollusc, excavating granite.

(Photograph by C. H. Snow; permission of Amer. Soc. Civil Engineers.)]

[Ill.u.s.tration: FIG. 106.--_Pholas_ sp., a mollusc, burrowing in sandstone. (Photograph by C. H. Snow; permission of Amer. Soc. Civil Engineers.)]

The "pearl-oyster" is not a true oyster, that is, not a member of the family to which the edible oysters belong, but it is a member of the same cla.s.s, that is, it is a bivalve mollusc. Pearls are obtained from a number of different "pearl-oysters," but the finest pearls and mother-of-pearl come from the tropical species _Meleagrina margaritifera_. This pearl-oyster "has an extensive distribution, being found in Madagascar, the Persian Gulf, Ceylon, Australia, Philippine Islands, South Sea Islands, Panama, West Indies, etc."

Mother-of-pearl is simply the inner lining of the sh.e.l.l, which is composed of numerous thin layers of carbonate of lime so arranged that the edges of the successive layers produce many fine striae very close together. The beautiful iridescence of this inner sh.e.l.l-lining is caused by the complicated diffraction and reflection (interference effects) of the light by the fine striae and the translucent superposed thin plates of sh.e.l.l material. Pearls are simply isolated deposits of sh.e.l.l material usually around some particle of foreign substance which has found lodging in the mantle-cavity. Sometimes small objects are purposely introduced into the sh.e.l.l in order to stimulate the formation of pearls. The pearl-fishers go out in boats and dive to the bottom, filling baskets with pearl-oysters. These are piled up in a bin and left to die and decompose. "When the flesh is pretty thoroughly disintegrated, it is washed away with water, great care being taken that none of the pearls loose in the flesh are lost. When the washing is concluded the sh.e.l.ls themselves are examined for pearls which may be attached to the interior of the valves." The princ.i.p.al pearl-fishery is that on the coast of Ceylon; pearl-fishing has been carried on here for over 2000 years.

[Ill.u.s.tration: FIG. 107.--_Martesia xylophaga_, a Pholad, in Panama mahogany. (Photograph by C. H. Snow; permission of Amer. Soc. Civil Engineers.)]

The ship-worm (_Teredo_) is an interesting member of this cla.s.s of bivalve molluscs, because of its unusual habits, and strangely modified body form. The teredo is long and worm-like in general appearance, with a small bivalve sh.e.l.l at one end and two elongated siphons at the other. The young teredo is a free-swimming ciliated embryo like the young of the other bivalve molluscs, but it soon settles on a piece of submerged wood, usually the pile of a wharf, or the bottom of a ship, and burrows into this wood. As it grows it enlarges and deepens its tube-like burrow, and lines it with a calcareous deposit. The burrow may be a foot long or longer, and when thousands of teredos attack a pile or the bottom of a ship, the wood soon becomes riddled with holes. These boring molluscs do great damage to wharves and ships. In Holland where they were first discovered they caused such injuries to the piles and other submerged wood which supported the dikes and sea-walls that they seriously threatened the safety of the country.

[Ill.u.s.tration: FIG. 108.--The giant yellow slug of California, _Ariolimax californica_. This slug reaches a length when outstretched of 12 inches. (From living specimen.)]

=Snails, slugs, nudibranchs and "sea-sh.e.l.ls"

(Gastropoda).=--TECHNICAL NOTE.--Pond-snails can be readily found clinging to submerged stems, leaves, or pieces of wood in almost any pond. Collect some and carry alive, in a jar of water, to the schoolroom. Observe the habits of these live snails in the school aquarium. Note the movements, the coming to the surface to breathe, the eating (by sc.r.a.ping the surface of the leaves with the "radula"

or tongue; provide fresh bits of cabbage or lettuce-leaves), the use of the feelers. Make drawings ill.u.s.trating these habits. Examine the sh.e.l.l; note that it is univalved, that is, composed of one piece. Do the whorls of all the sh.e.l.ls turn the same way? Make a drawing of the sh.e.l.l, naming such parts as the apex, spire (all the whorls taken together), the aperture, the columella (the axis of the spire), the lip (outer edge of the aperture), the lines of growth (parallel to the tip), the suture (the spiral groove on the outside). Examine the snail; note the character of the foot; note the protrusible tentacles or feelers, the eyes (dark spots at bases of the tentacles), the mouth, the respiratory opening (on right side of body in the edge of the mantle which protrudes beneath the lip when the snail's body is extended), the radula or ribbon-like tongue with fine teeth. Compare with the body of the mussel.

Slugs may be found during the day concealed under boards or elsewhere; they are nocturnal in habit. If specimens can be obtained, compare with the pond-snails, noting the absence of a sh.e.l.l, and the fleshy mantle on the dorsal surface near the head; note the presence of two pairs of tentacles (the eyes being at the tips of the second or hinder pair), and the respiratory pore. Note the streak of mucus left by the slugs in crawling about.

Some sea-sh.e.l.ls can be got from private collections of "curios" to ill.u.s.trate the variety of form of the univalve sh.e.l.ls.

Perhaps one-half of all the known species of molluscs are snails and slugs (fig. 108). Snails are either aquatic or terrestrial in habit, but in either case they (the true pulmonate snails) breathe not by means of gills, as do most of the other molluscs, but by means of a so-called "lung." This lung is a sac with an external opening on the right side of the body and with its inner surface richly furnished with fine blood-vessels. The exchange of gases between the blood and the outer air takes place through the thin walls of the blood-vessels.

Most snails which live in the water, as the pond-snails and the river-snails, have to come occasionally to the surface to breathe.

These fresh-water and land-molluscs which possess a lung-sac instead of gills const.i.tute the order Pulmonata. The pulmonate pond- and land-snails and slugs are vegetable feeders and where they occur in large numbers do much injury to vegetation. While the common pond-snails have but one pair of feelers, at the base of which are found the eyes, most of the land-snails and slugs have two pairs of "horns," the eyes being on the tips of the second pair. The lung-sac, besides serving as a breathing organ, also enables the snail to rise or sink according as the animal varies the size of the sac and consequently the amount of air in it. All the Pulmonata are hermaphroditic, each individual producing both sperm- and egg-cells.

The eggs of the pond-snail "are laid in gelatinous transparent capsules, half an inch to an inch in length, flattened and linear or oblong in outline. After a few snails have been kept a short time in a small vessel of water with their appropriate food, these egg-capsules may be looked for on the bottom and sides of the vessel or closely adherent to the stems or leaves of plants placed in the water. They are so transparent as to be easily overlooked." Young snails may be reared from these eggs.

There are other snails common in ponds, also called, like the pulmonate forms, pond-snails, which have gills and no lung-sac. These pond-snails belong to a different order of molluscs, and live on the bottom of the pond, crawling about in the soft mud and feeding on animal instead of vegetable food.

The sh.e.l.ls of the various kinds of snails vary much. In many of the land-snails the spiral is not spire-shaped or conical, but is flat. In some the whorls of the spiral run from left to right (dextral) when the sh.e.l.l is looked at with apex held toward one, while in others the whorls run from right to left (sinistral).

[Ill.u.s.tration: FIG. 109.--Three Pacific Coast nudibranchs; _Doris tuberculata_ (in lower left-hand corner), _Echinodoris_ sp. (upper one), and _Triopha modesta_ (at right). (From living specimens in a tide-pool on the Bay of Monterey, California.)]

Of the hosts of marine Gastropods we can notice only a few kinds. The nudibranchs (fig. 109) are a group of beautiful forms in which the sh.e.l.l is wholly wanting and the mantle is usually absent. The gills are thus exposed and are usually in the shape of delicate freely projecting tufts arranged in rows along the back. The body is often strikingly and variedly colored. These soft, naked "sea-slugs" live near the sh.o.r.e, creeping about among the rocks and seaweeds. About a thousand species of nudibranchs are known.

Among the sh.e.l.l-forming marine Gastropods there is great variety in the size and shape and coloring of the sh.e.l.ls. Many are beautifully colored and patterned; others are oddly and fantastically shaped. The cowries, or porcelain sh.e.l.ls, familiar in collections of ocean curiosities, have a large body whorl and a very short flat spire, and the brightly colored sh.e.l.l looks as if enamelled. Some of the coast tribes of Africa once used, and perhaps still use to some extent, cowries as money. The limpets (fig. 104) are among the most abundant of the seash.o.r.e molluscs, their low, broadly conical sh.e.l.ls being plentifully scattered over the rocks between tide-lines. The "oyster-drills" are Gastropods with odd spiny sh.e.l.ls which do much harm in oyster-beds by settling down on the oysters, boring holes through the sh.e.l.ls and eating the soft parts within. The helmet-sh.e.l.ls, from which sh.e.l.l cameos are cut, are composed of layers of sh.e.l.l material of different colors. Among the specially beautiful sh.e.l.ls are the cone-sh.e.l.ls, the olive-sh.e.l.ls, the ivory-sh.e.l.ls, etc.

=Squids, cuttlefishes, and octopi (Cephalopoda).=--TECHNICAL NOTE.--Small squids preserved in alcohol or formalin can be had of all dealers in biological supplies (see p. 453), and specimens should be examined.

The squids (fig. 110), cuttlefishes, octopi or "devil-fishes," and the three living species of _Nautilus_ const.i.tuting the cla.s.s Cephalopoda are very different from the other molluscs in appearance, and are in fact different in important structural characters. They can move swiftly, have strangely modified organs of prehension, strong biting mouth-parts, and eyes of very complex organization. They are the most highly organized molluscan forms, and their predaceous habits and the great size to which some of them attain have given them distinction among the fierce and dangerous creatures of the sea. They are all strictly marine in habitat, and are all carnivorous. Most of them have no sh.e.l.l, or where the sh.e.l.l is present it is internal in all but a very few forms. The tentacle-like arms or feet surrounding the mouth which occur in all the Cephalopods are provided with sucking organs or suckers, in some cases with a h.o.r.n.y toothed rim. These long, powerful, grasping, tentacular feet, with the suckers and five hooks, are very effective means of securing prey, and the pair of strong, sharp, cutting mandibles or beaks are equally effective in tearing to pieces. The eyes of the Cephalopods are almost as highly developed as those of the vertebrates. They are unusually large and staring, and add much to the terrifying appearance of the "devil-fishes." Cephalopods have the power of quickly changing color, because of the presence in the skin of many pigment-cells which can expand so as nearly to touch each other, thus producing a uniform tint over the whole body, or which can contract so as to destroy this uniformity of color. There are several sets of these color-carrying cells or chromatoph.o.r.es, each set of a color different from the others. The purpose of this change of color is protective, the animal being thereby able to make its color so harmonize with that of its immediate surroundings as to become indistinguishable.

There are two princ.i.p.al groups of Cephalopods, viz., the Decapods and the Octopods. The Decapods, as their name indicates, have ten feet or arms surrounding the mouth, and in them the body is usually elongate, containing a h.o.r.n.y "pen" or calcareous "bone." This group includes the cuttlefishes or sepias, from which are obtained sepia ink and the cuttlefish bone used to feed canary birds. The ink is a secretion which the cuttlefish discharges when attacked to create a cloud in the water and thus escape unperceived. The squids (_Loligo_) commonly used as bait by fishermen belong to the Decapoda. The two extra feet or arms which the Decapods have in addition to the eight possessed by the Octopods, differ from the others in being longer and slenderer and having suckers only on the distal extremities which are expanded into "clubs" (fig. 110).

[Ill.u.s.tration: FIG. 110.--The giant squid, _Ommatostrephes californica_.

(From specimen with body (exclusive of tentacles) four feet long, thrown by waves on sh.o.r.e of the Bay of Monterey, California.)]

The Octopods have a short, sac-like, sub-spherical body and neither external nor internal sh.e.l.l. To this group belong the famous devil-fishes (_Octopus_), whose strange and terrifying appearance combined with their frequently great size has furnished the basis for many a weird tale of the sea. _Octopi_ have been killed having tentacles more than 30 feet in length. The largest members of the cla.s.s, however, are probably the giant squids (belonging to the Decapoda) specimens of which have been captured with a body-length of twenty feet, and arms thirty-five feet long.

The beautiful paper sailor or argonaut (_Argonauta argo_), which secretes a thin sh.e.l.l (not h.o.m.ologous with the sh.e.l.l of the other molluscs) to protect her eggs, is a member of the Octopod group. In fine weather the argonauts sail in fleets on the surface of the ocean.