The fresh-water mussel which we have studied is an example of the bivalve molluscs. The members of this class 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 shell, but not anything like the degree of variety shown by the shells of the Gastropods.
The edible clams are of several different species. The hard-shell 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 shores, 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 shell partly exposed." The shells of this edible clam are white. The soft-shell 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 shore, 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 shells 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 shell, furnishing the gills with oxygen and the mouth with food, and then the water charged with carbonic acid and fæcal 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 When the tide ebbs the siphons are closed and partly withdrawn."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-shells which have become covered over with soil and vegetation. Such mounds are the old feasting-places of the early coast inhabitants, and the archæologist often finds in these "kitchen-middens," as they are called, various relics of the early natives of the continent.
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 californianus; in right-hand lower corner the black shells just above the large clam-shell, 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 class of molluscs. The oyster is carefully cultivated by man in many countries. It has its two shells or two shell-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."
Fig. 105.—Dactylus sp., a mollusc, excavating granite. (Photograph by C. H. Snow; permission of Amer. Soc. Civil Engineers.)
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 class, 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 shell, which is composed of numerous thin layers of carbonate of lime so arranged that the edges of the successive layers produce many fine striæ very close together. The beautiful iridescence of this inner shell-lining is caused by the complicated diffraction and reflection (interference effects) of the light by the fine striæ and the translucent superposed thin plates of shell material. Pearls are simply isolated deposits of shell material usually around some particle of foreign substance which has found lodging in the mantle-cavity. Sometimes small objects are purposely introduced into the shell 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 shells themselves are examined for pearls which may be attached to the interior of the valves." The principal pearl-fishery is that on the coast of Ceylon; pearl-fishing has been carried on here for over 2000 years.
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 class 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 shell 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.
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-shells" (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 scraping 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 illustrating these habits. Examine the shell; note that it is univalved, that is, composed of one piece. Do the whorls of all the shells turn the same way? Make a drawing of the shell, 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 shell, 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-shells can be got from private collections of "curios" to illustrate the variety of form of the univalve shells.
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 constitute 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 shells 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 shell is looked at with apex held toward one, while in others the whorls run from right to left (sinistral).
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 shell 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 shore, creeping about among the rocks and seaweeds. About a thousand species of nudibranchs are known.
Among the shell-forming marine Gastropods there is great variety in the size and shape and coloring of the shells. Many are beautifully colored and patterned; others are oddly and fantastically shaped. The cowries, or porcelain shells, familiar in collections of ocean curiosities, have a large body whorl and a very short flat spire, and the brightly colored shell 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 seashore molluscs, their low, broadly conical shells being plentifully scattered over the rocks between tide-lines. The "oyster-drills" are Gastropods with odd spiny shells which do much harm in oyster-beds by settling down on the oysters, boring holes through the shells and eating the soft parts within. The helmet-shells, from which shell cameos are cut, are composed of layers of shell material of different colors. Among the specially beautiful shells are the cone-shells, the olive-shells, the ivory-shells, 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 constituting the class 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 shell, or where the shell 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 horny 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 chromatophores, 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 principal 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 horny "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).
Fig. 110.—The giant squid, Ommatostrephes californica. (From specimen with body (exclusive of tentacles) four feet long, thrown by waves on shore of the Bay of Monterey, California.)
The Octopods have a short, sac-like, sub-spherical body and neither external nor internal shell. 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 class, 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 shell (not homologous with the shell 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.
The pearly nautilus (Nautilus pompilius) is a Cephalopod with four gills instead of two, as with the Decapoda and Octopoda, and is the only existing member of what was in the earlier times of the earth's history a large group of animals. The nautili live in rather shallow water usually creeping over the bottom feeding on small marine animals. They make a many-chambered spiral shell with its inner surface lined with beautiful pearly nacre.
CHAPTER XXIII
BRANCH CHORDATA: THE VERTEBRATES, ASCIDIANS, ETC.
The branch Chordata includes all the backboned animals or vertebrates, comprising the fishes, salamanders, frogs and toads, lizards, crocodiles, turtles and snakes, birds, and all the quadrupeds or mammals, and includes also a few small unfamiliar ocean animals which do not look at all like the backboned animals, but which agree with them in possessing a peculiar structure called the notochord. This notochord consists of a series or cord of cells extending longitudinally through the body from head to tail, above the alimentary canal and below the spinal nerve-cord. In all the vertebrates excepting a few low forms, the notochord while present in the young, is replaced in the adult by a segmented bony or cartilaginous axis, the spinal or vertebral column. But in the ascidians or sea-squirts (called also tunicates) it persists throughout life. In addition to this characteristic notochord, nearly all the Chordata are marked by the presence, either in embryonic or larval stages only, or else persisting throughout life, of a number of slits or clefts in the walls of the pharynx which serve for breathing, and which are called gill-slits.
Structure of the vertebrates.—As the backboned or vertebrate animals make up almost the whole of the branch Chordata, and as the few other chordates are animals the special structures of which we shall not undertake to study in this book, we may note here some of the other more obvious structural characteristics of the true vertebrates. The possession of a backbone or bony (sometimes cartilaginous) spinal column is the characteristic by which we distinguish them from the invertebrate or backboneless animals. Furthermore, all of the vertebrates possess an internal skeleton which is in most cases composed of bone, and is firm and strong. In some of the lower fishes, as the sharks and sturgeons, the skeleton is made up of cartilage, tough but not hard. The vertebrate skeleton consists typically of an axial portion comprising the spinal column and head, and of two pairs of appendages or limbs, variously developed as fins, wings, legs and arms. In some vertebrates these limbs are represented by mere rudiments, and in the lowest fish-like forms, the lancelets and lampreys, there is not the slightest trace of limbs. A part of the central nervous system, the spinal cord, runs longitudinally through the body on the dorsal side of the alimentary canal; the circulatory system is closed, the blood being always confined in the heart and in vessels called arteries, veins, and capillaries, and the blood is red in color owing to the presence of numerous red corpuscles or blood-cells. The nervous system is highly developed, with a large brain in all the typical forms, and with complex and usually highly efficient special sense-organs. Respiration is carried on by means of external gills, or by internal lungs which communicate with the outside through the mouth and nostrils. To the lungs and gills the blood is brought to be "purified," i.e., to give up its carbonic-acid gas and to take up oxygen.
Classification.—The Chordata are variously divided by zoologists into eight or ten classes, of which (in the eight-class system) the five classes[15] Pisces (fishes), Batrachia (batrachians), Reptilia (reptiles), Aves (birds), and Mammalia (mammals), belong to the true vertebrates. These classes will be considered in the five following chapters.
The remaining three classes include a number of strange marine forms which until recent years were considered as worms, but which are now known to be the nearest living allies of the earliest or primitive vertebrates. The relationship of these forms to early types is manifest, not in the appearance or structure of the adult stage, but only during embryonic or larval stages.
The ascidians.—The sea-squirts, or Ascidians, common on the seashore, compose one class of these primitive chordate animals. They possess a simple, sac-like body (fig. 111), fastened to the rocks by one end, the other being provided with two openings, one for the ingress and the other for the exit of water, a strong current of which flows constantly through the body. By means of this current the ascidian obtains food. Usually sea-squirts live together in large colonies, and in some cases a number of individuals enclose themselves in a common gelatinous mass, forming what is called a compound ascidian.
The ascidian when born is a tiny, free-swimming, tadpole-like creature with a slender finned tail. It swims about freely for only a few hours, however, soon attaching itself to a rock, and in its further development becoming degenerate. It loses its tail and with it the short notochord possessed by the larva; the eye and the auditory organ are lost, and the nervous system and alimentary canal become much reduced and simplified. Sea-squirts in their adult stage are very simple degenerate animals, with low functional development, yet their embryonic and larval conditions show a considerable degree of structural specialization, and the presence of the notochord in these early stages reveals their affinity with the backboned animals.
CHAPTER XXIV
BRANCH CHORDATA (Continued): CLASS PISCES (THE FISHES)
THE GOLDEN SUNFISH OR PUMPKIN SEED (Apomotis sp.)
Technical Note.—The species of sunfish named, or some closely related species, can be obtained in any brook or stream in the United States. Gibbosus lives in all streams north of Dubuque, Chicago, Pittsburg, and along the eastern coast north of Charleston. Closely allied species live in all the other parts of the country except in the higher Rocky Mountains west of Bismarck, Pueblo, and Santa Fe. One species is found in the streams of California, but none occurs in Washington or Oregon. In the few places where a sunfish cannot be had, any species of bass or perch may be used. Sunfish live in ponds and sluggish streams in deep holes under a log or at the foot of a stump. They take eagerly a hook baited with a worm, or they may be caught in nets. When sunfish cannot be kept fresh for study in class, specimens may be preserved in alcohol or 4% formalin. But if possible to keep some alive for a time in a jar or tub with plenty of fresh water, the colors of the living fish, together with its manner of swimming and mode of breathing, can be observed.
External structure[16] (fig. 112).—Examine the general configuration and make-up of the body. Note the deep, laterally flattened trunk and paddle-like tail. The head is closely fitted to the trunk without any neck. Note that the body is thickly covered with firm, hard scales, arranged like the shingles on a roof. Remove one of these scales and examine it under a hand lens. What sort of an edge has it? Such a scale is said to be ctenoid.
The body of the sunfish terminates behind in the caudal fin, a series of cartilaginous rays connected by thin skin and attached to a bony plate at the end of the backbone. Along the median dorsal line will be noted another fin composed anteriorly of spines and posteriorly of soft rays jointed and branched. This is the dorsal fin. How many spines has it? Anterior to the caudal fin on the ventral surface is a median unpaired anal fin. How many spines has it? Anterior to the anal fin are the ventral fins, while on the sides of the body back of the head in a line with the mouth are found the pectoral fins. The ventral fins, attached to a rudimentary pelvis, correspond to the hind legs of the other vertebrates. The pectoral fins, attached to the shoulder girdle, correspond to the arms. In front of the anal fin note a small pit-like opening, the opening from the kidneys and reproductive organs, and just anterior to this a large aperture, the anus. At the anterior end of the head note the broad mouth, surrounded by a complicated system of bones. Note the large eyes surrounded by a series of small bones, the orbital chain. Just anterior to the eyes are two pairs of openings, one pair of each side opening into a closed sac. What are these openings? Note the presence of various bones on the side of the head, each covered with a thin layer of skin. These are membrane bones, characteristic of fishes. Are there any external ears in the fish? Examine the inside of the mouth. Is there a tongue? If so, of what character? Are there teeth? If so, where are they situated?
Note along each side extending to the base of the tail a line of modified scales, on each scale a little mucous tube, the whole series constituting the lateral line. These scales are intimately associated with a large nerve (the vagus), and probably serve an important part, not yet clearly understood, in the life of the fish.
Lift up the flap in front of one of the pectoral fins. This is the opercular flap which covers the gills that lie beneath. Bend this forward and find four gill-arches, each with its double fringe of gills. Note the gill-rakers, short and blunt, on the first gill-arch. Note also on the under side of the flaps turned back, delicate red gill-like structures covered by a membrane. These are the false gills or pseudo-branchiæ, larger in most fishes than in the sunfish. The gills in the fish subserve the same function as the gills of the crayfish, that of purifying the blood by eliminating carbonic-acid gas from it and taking up oxygen from the air mixed with or dissolved in the water. Organs subserving the same purposes in different kinds of animals as, for example, the gills in fish and in crayfish, are called analogous structures. But there is an important morphological difference between the fish's gills and the gills of the crayfish. In the latter animal they are outgrowths of the basal segments of the walking-legs; in the fish they are outgrowths from the alimentary canal. The internal gills of the young toad (tadpole) arise in the same way as those of a fish. Structures which are identical in their origin, like the gills of tadpole and fish, are called homologous structures.
Make a drawing of the sunfish from a lateral aspect, showing the external parts named.
Internal Structure.—Technical Note.—Insert one point of the scissors a little to one side of the anus and cut dorsally on the left side of the body to the backbone. Now cut anteriorly from the anus along the ventral wall to where the jaws unite, and cut, also anteriorly, along the dorsal wall until the left side of the body can be removed. Bend the opercular flap backward over the eye and pin the entire fish, uncut side down, to the bottom of the dissecting-pan, covering it with water.
The above operation will have severed the large powerful muscles forming the body-wall and extending along the sides. Note a membranous sac completely filling a large dorsal cavity. This is the swim-bladder, a float filled with air which tends to give the fish the same weight as the water it displaces. It arises as a diverticulum from the alimentary canal, but soon becomes permanently shut off from it. Beneath the swim-bladder is a large cavity filled with various organs, collectively known as the viscera. In vertebrate animals the cavity which contains the viscera is generally called the peritoneal cavity. It is lined by the peritoneum, a delicate membrane, part of which is deflected as the mesentery over the alimentary canal and the other organs, thus suspending them all from the dorsal wall. Note in the anterior end of the peritoneal cavity a large bi-lobed gland, the liver, red in fresh, yellowish in alcoholic specimens. Its function, like that of the liver of the toad, is to store up nutriment for the blood and to secrete a digestive fluid called bile. Behind the liver note a long, convoluted tube. What is this tube? Unfold this tube, separating it from its enveloping membrane, the mesentery. Thrust a probe down the throat and note that it passes into a thick-walled sac, the stomach. The mouth and gill-slits open into the front part of the alimentary canal called the pharynx, which leads by a short tube, the œsophagus, into the stomach. Note the large, thickened portion of the alimentary canal leading from the stomach. This is the pylorus, and to its walls are attached a number of finger-like projections, the pyloric cæca. The pyloric cæca secrete a fluid which is poured into the alimentary canal and which assists in the process of digestion somewhat as does the secretion from the pancreas of the toad. From the pylorus, passing backwards in one or two loops, is the small intestine. Trace this to its exit. Lying within the mesentery near the posterior end of the body-cavity note a small red glandular mass, the spleen.
At the anterior end of the body in front of the liver and between the sets of gills note the small pericardial cavity within which is contained the heart. The pericardial cavity is separated from the peritoneal cavity by a thick muscular wall against which the liver abuts. The heart consists of four parts. The posterior part is a thin-walled reservoir, the sinus venosus, into which blood enters through the jugular vein from the head and through the cardinal vein from the kidney. From the sinus venosus it passes forward into a large chamber, the auricle. Next it flows into the ventricle, where, by the contraction of the walls, rhythmical pulsations force it into the conus arteriosus, thence into the ventral aorta, and lastly into the gills, where it is purified. After passing through the capillaries in the fine gill-filaments it is again collected, now pure, by paired arteries from each pair of gills, which arteries unite to form the dorsal aorta extending backward just below the backbone to the end of the tail. From the dorsal aorta a pair of arteries, the subclavian, are given off to the pectoral fins. At this point two other arteries branch off ventrally, the first being the cardiac artery, which distributes blood to the stomach and pyloric cæca. The second divides into several long mesenteric arteries supplying blood to all parts of the intestine and spleen. In the caudal region blood is taken up through the caudal vein and carried forward to the kidneys. These strain out the impurities arising from waste of tissues, after which the blood is carried back to the sinus venosus through the cardinal vein. From the intestine it is gathered into the large portal vein as in the toad. The portal vein carries blood to the liver, where nutriment may be stored up, and from thence it flows back to the sinus venosus through a very short thin-walled vessel, the hepatic sinus.
The kidneys, more or less united in one mass, lie in the posterior part of the body-cavity along the dorsal wall. Note running from each side of the kidney a ureter which unites with its fellow and opens into a small urinary bladder which discharges through a small opening immediately back of the anus.
The reproductive organs lie below the swim-bladder near the posterior end of the body-cavity. If the fish are caught in the spring, the greater part of the body-cavity of the female is found to be filled with small eggs. When mature, these eggs are deposited by the mother fish in the gravel of the stream-bed where they are fertilized by the sperm-cells poured over them by the male and left floating in the water.
The nervous system of fishes is best studied in a specimen treated with nitric acid. Carefully remove the roof of the skull, thereby exposing the brain. Most anteriorly make out, as in the toad, the paired olfactory lobes. These are attached by long stalks to the cerebrum or forebrain, which is followed by two large hollow lobes, the midbrain or optic lobes. Behind the midbrain is the cerebellum. Following the cerebellum is the elongate medulla oblongata, which tapers backward into the spinal cord. How far backward does the spinal cord extend? On each side of the brain-case about opposite the cerebellum are located the auditory organs, each consisting of three semicircular canals which lie in different planes, and of the vestibule. These parts are filled with liquid, and suspended in the liquid in the vestibule are small calcareous bodies called otoliths or ear-stones. Running out beneath from the midbrain are the optic nerves, which cross, the left one connected with the right eye, the right one with the left eye. From each side of the medulla oblongata there is given off a large nerve, the vagus, which sends branches to the lateral line organs on either side, and extends backward to the stomach and viscera.
For further study of the nervous system see Parker's "Zootomy," pp. 122-130.
Make a drawing of the nervous system as worked out.
Technical Note.—To make a good skeleton immerse a fresh or preserved specimen for some time in a hot soap solution. When the muscles have commenced to soften remove the body from the solution, pick the flesh away, and leave to dry.
Note that the main axis of the skeleton is composed of vertebræ placed end to end. How many vertebræ are there? What vertebræ bear ribs? The ribless ones beyond the body-cavity are called caudal vertebræ. Note the interspinal bones which support the fins, with large muscles on either side to control their action. Note that the group of bones supporting the pectoral fin is attached to the back of the brain-case and makes up the shoulder girdle. The ventral fins are attached to a rudimentary pelvic girdle, attached in front to the shoulder girdle, as the shoulder girdle is in turn attached to the skull. It will be seen that the sunfish has no neck and we may say, also, no back. Its skeleton consists only of a tail attached to the skull. The brain-case is made up of a number of bones closely joined together. From it is suspended the lower jaw, which comprises a number of bones but loosely attached to each other. Overlying these is the system of membrane bones already mentioned, including the opercle or gill-cover.
For a detailed study of the fish-skeleton see Parker's "Zootomy," pp. 86-101, or Parker and Haswell's "Zoology," vol. ii. pp. 183-195.
Life-history and habits.—The sunfish or "pumpkin-seed" lives in quiet corners of the brooks and rivers, preferably under a log or at the root of an old stump. It is a beautiful fish, shining "like a coin fresh from the mint." Its body is mottled golden, orange and blue, with metallic lustre, darker above, pale or yellowish below. Its fins are of the same color. The tip of its opercle is prolonged like an ear and jet black in color, with a dash of bright scarlet along its lower edge. Nearly all the thirty species of sunfish found in the United States have this black ear, but some have it long, some short, and in some it is trimmed with yellow or blue instead of scarlet.
The sunfish lays its eggs in the spring in a rude nest it scoops in the gravel, over which it stands guard with its bright fins spread, looking as big and dangerous as possible. When thus employed it takes the hook savagely, perhaps regarding the worm as a dangerous enemy. The young fishes soon hatch, looking very much like their parents, although more transparent and not so brightly colored. They grow rapidly, feeding on insects and other small creatures, and reach their growth in two or three years. They do not wander far and never willingly migrate. Students should verify this account on the different species. A more exact study of the nests of the different species and the fishes' defence of them would be a valuable addition to our knowledge. The most striking traits of the habits of this fish are its vivacity and courage; it reveals its great muscular strength when captured. The sexes are similar in appearance and both defend the nest alike.
OTHER FISHES.
Fishes constitute the largest class of vertebrate animals and are to be found everywhere in ponds, streams, or ocean. About 15,000 species of fish are known, of which 3,000 live in North America. The largest of all fishes is the basking shark (Cetorhinus), which reaches a length of thirty-six feet. The smallest is the dwarf goby (Mistichthys), less than half an inch long, found in Luzon, one of the Philippine Islands. Between these extremes is every variety in size, form, and relative proportions. The body, for example, may be greatly elongated and almost cylindrical as in the eels; or long and flattened from side to side as in the ribbon-fishes; or the head may be very large, wider and higher than the rest of the body as in the anglers, or may have a great beak as in the sword-fish.
Body form and structure.—When we consider the fish as a whole, we find first a body formed for progression in the water, the typical fish being pointed at each end (the shorter point in front), and having the sides flattened, the back and belly rather narrow, and the motive power located in the fin on the tail. From this typical form diverge all conceivable variations, adaptations to every sort of fish life.
Most fishes have the body covered with scales, although many have the skin naked or covered with small scales so hidden in the skin as to be hardly visible. The scales are small horny or bony plates which fit into small pockets or folds of the skin, and are usually arranged shingle-fashion, overlapping each other. They are of various shapes, mostly classified as of three kinds, namely, squarish enamelled scales called ganoid, roundish smooth-edged called cycloid, and roundish tooth-edged called ctenoid.
The skeleton of the fish is relatively complex. Its bones are comparatively soft, having little lime in them, indeed in many cases they are mere cartilage. The vertebral column is made of twenty-four vertebræ in the typical fishes, the number in the others being variously increased, or sometimes diminished. These vertebræ are of two classes, abdominal or body, and caudal or tail vertebræ. The former have a neural arch which encloses the spinal cord and from which projects a spine. Below, the processes spread apart, surrounding the kidneys and partly enclosing the air-bladder. To these processes ribs are loosely attached. The caudal vertebræ have no ribs and leave no room below for viscera. Their lower arch (hæmal), similar to the dorsal (neural) arch, surrounds a blood-vessel. The fins of a fish are composed of bony rods or rays joined by membrane. Some of these rays may be unbranched and unjointed, being then known as spines, and usually occupy the front part of the fin. Other rays are made up of little joints and are usually branched toward their tip. Such ones are called soft rays. Soft rays make up the greatest part of most fins. The vertical fins are on the middle line of the body. These are the dorsal above, anal below, and caudal forming the end of the tail. The paired pectoral and ventral fins are ranged one on each side corresponding to the arms and legs of higher animals. The pectoral fin or arm is fastened to a series of bones called the shoulder girdle. These bones do not correspond to those in the shoulder girdle of the higher animals, and the various parts in the two structures are differently named. The uppermost bone of the shoulder girdle is usually attached to the skull. To the lowermost is attached the rudimentary pelvis, which supports the hinder limb or ventral fin. Usually the pelvis is farther back and loose in the flesh, but sometimes it is placed far forward, being occasionally attached at the chin.
The head contains the various bones of the cranium, usually closely wedged together and not easily distinguished. The jaws are each made of several pieces; the lower one is suspended from the skull by a chain of three flat bones. The jaws may bear any one of a great variety of forms of teeth or no teeth at all, and any of the bones of the mouth-cavity and throat may have teeth as well. On the outside of the head are numerous bones called membrane bones, because they are made up of ossified membrane. The most important of these is the opercle or gill-cover. Within are the tongue with the five gill-arches attached to it below and to the floor of the skull above, the last arch being usually modified to form the pharyngeal jaw.
The stomach may be a blind sac with entrance and exit close together, or it may have the form of a tube or siphon. At its end are often found the large glandular tubes called pyloric cæca which secrete a digestive fluid; and to its right side is attached the red spleen. The liver is large, having usually, but not always, a gall-bladder; it pours its secretion into the upper intestine. In fishes which feed on plants the intestine is long, but it is short in those which eat flesh, because flesh is digested in the stomach, not in the intestines. The kidney is usually a long slender forked gland showing little variation. The egg-glands differ greatly in different sorts of fishes, the size and number of eggs varying equally. The air-bladder is a lung which has lost both lung structure and respiratory function, being simply a sac filled with gas secreted from the blood, and lying in the upper part of the abdominal cavity. It is subject to many variations. In the gar pike, bow-fin and the lung-fishes of the tropics, the air-bladder is a true lung used for breathing and connected by a sort of glottis with the œsophagus. In others it is rudimentary or even wholly wanting, while in still others its function as an air-sac is especially pronounced, and in many it is joined through the modified bones of the neck to the organ of hearing.
The blood of the fish is purified by circulation through its gills. These are a series of slender filaments attached to bony arches. Among them the blood flows in and out, coming in contact with the water which the fish takes in through its mouth and which passes across the gills to be expelled through the gill-openings. The blood is received from the body into the first chamber of the heart, a muscular sac called the auricle. From here it passes into the ventricle, a chamber with thicker walls, the contraction of which sends it to the gills, thence without return to the heart it passes over the body. The circulation of blood in fishes is slow, and the blood, which receives relatively little oxygen, is cold, being but little warmer than the water in which the individual fish lives.
Inside the cranium or brain-case is the brain, small and composed of ganglia which are smooth at the surface and contain little gray matter. At the posterior end of the brain is the thickened end of the spinal cord, called the medulla oblongata. Next overlapping this is the cerebellum, always single. Before this lie the largest pair of ganglia, the optic lobes or midbrain, round, smooth, and hollow. From the under side of these, nerves run to the eyes with or without a chiasma or crossing. In front of the optic lobes and smaller than them is the cerebrum or forebrain, usually of two ganglia but sometimes (in the sharks) united into one. In front of these are the small olfactory lobes which send nerves to the nostrils.
The sense organs are well developed. The sense of touch has in some fishes special organs for its better effectiveness. For instance certain fin-rays in some fishes, or, as in the catfish, slender, fleshy, whip-like processes on the head, are developed as feelers or special tactile organs. Other fishes, the sucker and loach for example, have specially sensitive lips and noses with which they explore their surroundings. The sense of taste does not seem to be well developed in this group. Taste-papillæ are often present in small numbers on the tongue or on the palate. The sense of smell is good. The olfactory organs, one on each side of the head, are hollow sac-like depressions, closed at the rear. In most cases each sac has two openings or nostrils. The sense of hearing is not very keen. The ears are fluid-filled sacs buried in the skull, and without external or (except in a few cases) internal opening. Fishes are far more sensitive to sudden jars or sudden movements than to any sound. They possess what is generally believed to be a special sense organ not found in other animals. This is the lateral line which extends along the sides of the body and which consists of a series of modified scales (each one with a mucous channel) richly supplied with nerves. The eyes are usually large and conspicuous. They differ mainly from the eyes of other vertebrates in their myopic spherical crystalline lens, made necessary by the density of the medium in which fishes live. There are usually no eyelids, the skin of the body being continuous but transparent over the eyes. Being near-sighted, fishes do not discriminate readily among forms, their special senses fitting them in general to distinguish motions of their enemies or prey rather than to ascertain exactly the nature of particular things.
The colors of fishes are in general appearance protective. Thus most individuals are white on the belly, mimicking the color of the sky to the enemy which pursues them from below. Seen from above most of them are greenish, like the water, or brownish gray and mottled, like the bottom. Those that live on sand are sand-colored, those on lava black, and those among rose-red sea-weeds bright red. In many cases, especially among kinds that are protected by their activity, brilliant colors and showy markings are developed. This is especially true among fishes of the coral reefs, though species scarcely less brilliant are found among the darters of our American brooks.
Among fresh-water fishes bright colors, crimson, scarlet, blue, creamy white, are developed in the breeding season, the then vigorous males being the most highly colored. Many of the feeble minnows even become very brilliant in the nuptial season of May and June. Color in fishes is formed by minute oil-sacs on the scales, and it often changes quickly with changes in the nervous condition of the individuals.
Development and life-history.—The breeding habits of fishes are extremely varied. Most fishes do not pair, but in some cases pairing takes place as among higher animals. Ordinarily fishes lay their eggs on the bottom in shallow water, either in brooks, lakes, or in the sea. The eggs of fishes are commonly called spawn, and egg-laying is referred to as spawning. The spawn of some fishes is esteemed a special food delicacy. Spring is the usual time of spawning, though some fishes spawn in summer and some even in winter; generally they move from their usual haunts for the purpose. The eggs of the different species vary much in size, ranging from an inch and a half in diameter (barn-door skate) down to the tiniest dots, like those of the herring. The number of eggs laid also varies greatly. The trout lays from 500 to 1,000, the salmon about 10,000, the herring 30,000 to 40,000, and some species of river fish 500,000, while certain flounders, sturgeons, and others each lay several millions of eggs. The adults rarely pay any attention to the eggs, which are hatched directly by the heat of the sun or by heat absorbed from the water. The length of incubation varies much. When the young fish leaves the egg-shell it carries, in the case of most species, a part of the yolk still hanging to its body. Its eyes are very large, and its fins are represented by thin strips of membrane. It usually undergoes no great changes in development from the first, resembling the adult except in size. But some of the ocean fishes show a metamorphosis almost as striking as that of insects or toads or frogs.
Some fishes build nests. Sticklebacks build elaborate nests in the brooks and defend them with spirit. Sunfishes do the same, but the nests are clumsier and not so well cared for.
The salmon is the type of fishes which run up from the sea to lay their eggs in fresh water. The king salmon of the Columbia River, for example, leaves the sea in the high waters of March and ascends without feeding for over a thousand miles, depositing its spawn in some small brook in the fall. After making this long journey to lay the eggs, the salmon become much exhausted, battered and worn, and are often attacked by parasitic fungi. They soon die, probably none of them ever surviving to lay eggs a second time.
Classification.—A fish is an aquatic vertebrate, fitted to breathe the air contained in water, and never developing fingers and toes. Accepting this broad general definition we find at once that there are very great differences among fishes. Some differ more from others than the ordinary forms differ from rabbits or birds. So although we have entitled this chapter as if all fishes belonged to the class Pisces, we cannot arrange them satisfactorily in less than three classes.
The lancelets (Leptocardii).—The lowest class of fish-like animals is that of the lancelets, the Leptocardii. These little creatures, translucent, buried in the sand, of the size and form of a small toothpick, are fishes reduced to their lowest terms. They have the form, life, and ways of a fish, but no differentiated skull, brain, heart, or eyes. Moreover they have no limbs, no jaws, no teeth, no scales. The few parts they do have are arranged as in a fish, and they show something in common with the fish embryo. Lacking a distinct head, the lancelets are put by some zoologists in a group called the Acrania, as opposed to the Craniata, which includes all the other vertebrates. Lancelets have been found in the North Atlantic and Mediterranean, on the west coast of North America, on the east coast of South America and on the coasts of Japan, Australia, New Zealand, the East Indies and Malayan Islands. The best-known members of the group belong to the genus Amphioxus. There are but one to two other genera in the class.