The lampreys and hag-fishes (Cyclostomata).—The next class of fish-like animals is that of the lampreys (fig. 113) and hag-fishes, the Cyclostomata. The lampreys and hags are easily distinguished from the true fishes by their sucking mouth without jaws, their single median nostril, their eel-like shape and lack of lateral appendages or paired fins. The hag-fishes (Myxine), which are marine, attach themselves by means of a sucker-like mouth to living fishes (the cod particularly), gradually scraping and eating their way into the abdominal cavity of the fish. These hags or "borers" "approach most nearly to the condition of an internal parasite of any vertebrate." The lampreys, or lamprey-eels as they are often called because of their superficial resemblance to true eels, are both marine and fresh-water in their habitat, and most of them attach themselves to live fishes and suck their blood. They also feed on crustacea, insects, and worms. The brook-lamprey, Lampetra wilderi, is never parasitic. It reaches its full size in larval life and transforms simply for spawning. The sea- and lake-lampreys ascend small fresh-water streams when ready to lay their eggs, few living to return. Sometimes small piles of stones are made for nests. The young undergo a considerable metamorphosis in their development. The largest sea-lampreys reach a length of three feet. The common brook-lampreys are from eight to twelve inches long only.
The true fishes (Pisces).—All the other fish-like animals are grouped in the class Pisces. They are characterized, when compared with the lower fish-like forms just referred to, by the presence of jaws, shoulder girdle, and pelvic girdle. The class includes both the cartilaginous and bony fishes, and is divided into three sub-classes, namely, the Elasmobranchii, including the sharks, rays, skates, torpedoes, etc., the Holocephali, including the chimæras (a few strange-bodied forms), and the Teleostomi, including all the other fishes, as the trout, catfishes, darters, bass, herring, cod, mackerel, sturgeons, etc., etc.
The sharks, skates, etc. (Elasmobranchii).—The sharks and skates are characterized by the possession of a skeleton composed of cartilage and not bone, as in the bony fishes; they have no operculum; their teeth are distinct, often large and highly specialized, and their eggs are few and very large. There are two principal groups among Elasmobranchii, viz., the sharks, which usually have an elongate body, and always have the gill-openings on the sides, and the rays or skates, which have a broad flattened body with the gill-openings always on the under side. All the members of both groups are marine. The sharks are active, fierce, usually large fishes, which live in the surface-waters of the ocean and make war on other marine animals, all of the species except half a dozen being fish-eaters. The shark's mouth is on the under side of the usually conical head, and the animal often turns over on its back in order to seize its prey. The largest American sharks, and the largest of all fishes, are the great basking-sharks (Cetorhinus), which reach a length of nearly forty feet. They get their name from their habit of gathering in numbers and floating motionless on the surface. They feed chiefly on fishes.
The hammer-headed sharks (Sphyrna) are odd sharks which have the head mallet or kidney shaped, twice as wide as long, the eyes being situated on the ends of the lateral expansions of the head. The man-eating or great white sharks (Carcharodon) are nearly as large as the basking-sharks, and are extremely voracious. They will follow ships for long distances for the refuse thrown overboard. They do not hesitate to attack man. Among the more familiar smaller sharks are the dog-fishes and sand-sharks of our Atlantic coast.
The rays and skates are also carnivorous, but are with few exceptions sluggish, lying at the bottom of shallow shore-waters. They feed on crabs, molluscs, and bottom-fishes. The small common skates, "tobacco-boxes" (Raja erinacea) (fig. 114), about twenty inches long, and the larger "barn-door skates" (R. lævis), are numerous along the Atlantic coast from Virginia northward. Especially interesting members of this group, because of the peculiar character of the injuries produced by them, are the sting-rays and torpedoes or electric-rays. The sting-rays (Dasyatis) have spines near the base of the tail which cause very painful wounds. The torpedoes (Narcine) have two large electrical organs, one on each side of the body just behind the head, with which they can give a strong electric shock. "The discharge from a large individual is sufficient to temporarily disable a man, and were these animals at all numerous they would prove dangerous to bathers." Very different from the typical rays in external appearance are the saw-fishes (Pristis pectinatis) which belong to this group. The body is elongate and shark-like, and has a long saw-like snout. This saw, which in large individuals may reach a length of six feet and a breadth of twelve inches, makes its owner formidable among the small sardines and herring-like fishes on which it feeds. The saw-fishes live in tropical rivers, descending to the sea.
The bony fishes (Teleostomi).—The bony or true fishes are distinguished from the lampreys and sharks and rays by having in general the skeleton bony, not cartilaginous, the skull provided with membrane bones, and the eggs small and many. In this group are included all the fishes of our fresh-water lakes, ponds, and streams as well as most of the marine forms. Fish life, being spent under water, is not familiar to most of us, and beginning students are rarely helped enough in getting acquainted with the different kinds and the interesting habits of fishes. But they offer a field of study which is really of unusual interest and profit. We can refer in the following paragraphs to but few of the numerous common and readily found kinds, and to these but briefly.
Closely related to the sunfish, studied as example of the bony fishes, are the various kinds of bass, as the "crappie" (Pomoxis annularis), the calico bass (P. separoides), the rock-bass (Ambloplites rupestris) and the large-mouthed and small-mouthed black bass (Micropterus salmoides and M. dolomieu respectively). All the members of this sunfish and bass family are carnivorous fishes especially characteristic of the Mississippi valley.
Another family of many species especially common in the clear, swift, and strong Eastern rivers is that of the darters and perches. The darters are little slender-bodied fishes which lie motionless on the bottom, moving like a flash when disturbed and slipping under stones out of sight of their enemies. Some are most brilliantly colored, surpassing in this respect all other fresh-water fishes.
Unlike the sunfishes and darters are the catfishes, composing a great family, the Siluridæ. The catfish (Ameiurus) gets its name from the long feelers about its mouth; from these feelers also come its other names of horned pout, or bull-head. It has no scales, but its spines are sharp and often barbed or jagged and capable of making a severe wound.
Remotely allied to the catfish are the suckers, minnows, and chubs, with smooth scales, soft fins and soft bodies and the flesh full of small bones. These little fish are very numerous in species, some kinds swarming in all fresh water in America, Europe, and Asia. They usually swim in the open water, the prey of every carnivorous fish, making up by their fecundity and their insignificance for their lack of defensive armature. In some species the male is adorned in the spring with bright pigment, red, black, blue, or milk-white. In some cases, too, it has bony warts or horns on its head or body. Such forms are known to the boys as horned dace.
Most interesting to the angler are the fishes of the salmon and trout (fig. 115) family, because they are gamy, beautiful, excellent as food and above all perhaps because they live in the swiftest and clearest waters in the most charming forests. The salmon live in the ocean most of their life, but ascend the rivers from the sea to deposit their eggs. The king salmon (Oncorhynchus tschawytscha) of the Columbia goes up the great river more than a thousand miles, taking the whole summer for it, and never feeding while in fresh water. Besides the different kinds of salmon, the black-spotted or true trout, the charr or red-spotted trout of various species, the whitefish (Coregonus), the grayling (Thymallus signifer) and the famous ayu of Japan belong to this family.
In the sea are multitudes of fish forms arranged in many families. The myriad species of eels agree in having no ventral fins and in having the long flexible body of the snake. Most of them live in the sea, but the single genus (Anguilla) or true eel which ascends the rivers is exceedingly abundant and widely distributed. Most eels are extremely voracious, but some of them have mouths that would barely admit a pin-head. The codfish (Gadus callarias) is a creature of little beauty but of great usefulness, swarming in all arctic and subarctic seas. The herring (Clupea harengus), soft and weak in body, are more numerous in individuals than any other fishes. The flounders (fig. 116) of many kinds lie flat on the sea-bottom. They have the head so twisted that the two eyes occur both together on the uppermost side. The members of the great mackerel tribe swim in the open sea, often in great schools. Largest and swiftest of these is the sword-fish (Xiphias gladius), in which the whole upper jaw is grown together to form a long bony sword, a weapon of offence that can pierce the wooden bottom of a boat.
Many of the ocean fishes are of strange form and appearance. The sea-horses (Hippocampus sp.) (fig. 117) are odd fishes covered with a bony shell and with the head having the physiognomy of that of a horse. They are little fishes rarely a foot long, and cling by their curved tails to floating seaweed. The pipefish (Syngnathus fuscum) is a sea-horse straightened out. The porcupine-fishes and swellfishes (Tetraodontidæ) have the power of filling the stomach with air which they gulp from the surface. They then escape from their pursuers by floating as a round spiny ball on the surface. The flying-fishes (Exocœtus) leap out of the water and sail for long distances through the air, like grasshoppers. They cannot flap their long pectoral fins and do not truly fly; nevertheless they move swiftly through the air and thus escape their pursuers. In its structure a flying-fish differs little from a pike or other ordinary fish.
For an account of the fishes of North America see Jordan's "Manual of Vertebrates," eighth edition, pp. 5-173, and Jordan and Evermann's "Fishes of North and Middle America," where the 3,127 species known from our continent are described in detail with illustrative figures.
Habits and adaptations.—The chief part of a fish's life is devoted to eating, and as most fishes feed on other fishes, all are equally considerably occupied in providing for their own escape.
In general the provisions for seizing prey are confined to sharp teeth and the strong muscles which propel the caudal fin. But in some cases special contrivances appear. In one large group known collectively as the "anglers" the first spine of the dorsal fin hangs over the mouth. It has at its tip a fleshy appendage which serves as a bait. Little fishes nibble at this, the mouth opens, and they are gone. In the deep seas, many fishes are provided with phosphorescent spots or lanterns which light up the dark waters, and enable them to see their prey. In storms these lantern-fishes sometimes lose their bearings and are thrown upward to the surface.
In general the more predatory in its habits any fish is the sharper its teeth, and the broader its mouth. Among brook-fishes the pickerel has the largest mouth and the sharpest teeth. It has been called a "mere machine for the assimilation of other organisms." The trout has a large mouth and sharp teeth. It is a swift, voracious, and predatory fish, feeding even on its own kind. The sunfish is less greedy and its mouth and teeth are smaller, though it too eats other fish.
As means of escape, most fishes depend on their speed in swimming. But some hide among rocks and weeds, disguising themselves by a change in color to match their surroundings. Others, like the flounders and skates, lie flat on the bottom. Still others retreat to the shallows or the depths or the rock-pools or to any place safer than the open sea. Some are protected by spines which they erect when attacked. Some erect these spines only after they have been swallowed, tearing the stomach of their enemy and killing it, but too late to save themselves. Again in some species the spines are armed with poison which benumbs the enemy. Sometimes an electric battery about the head or on the sides gives the biting fish a severe shock and drives him away. Such batteries are found in the electric rays or torpedo, in the electric eel of Paraguay, the electric catfish of the Nile, the electric stargazer and other fishes.
Some fishes are protected by their poor and bitter flesh. Some have bony coats of mail and sometimes the coat of mail is covered with thorns, as in the porcupine-fish. This fish and various of its relatives have the habit of filling the stomach with air when disturbed, then floating belly upward, the thorny back only within reach of its enemies.
Fig. 118.—The remora, or cling fish, Remoropsis brachyptera. Note sucker on top of head. (After Goode.)
Many species (cling fishes) attach themselves to the rocks by a fleshy sucking-disk. Some (Remora) (fig. 118) cling to larger fishes by a strange sucking-disk on the head, a transformed dorsal fin, being thus shielded from the attacks of fish smaller than their protectors. Some small fishes seek the shelter of the floating jellyfishes, lurking among their poisoned tentacles. Others creep into the masses of floating gulf-weed. Some creep into the shell of clams and snails. In the open channel of a sponge, the mouth of a tunicate and in similar cavities of various animals, little fishes may be found. A few fishes (hag-fishes) are parasitic on others, boring their way into the body and devouring the muscles with their rasp-like teeth.
Some fishes are provided with peculiar modifications of the gills which enable them to breathe for a time out of water. Such fish have the pectoral fins modified for a rather poor kind of locomotion on land, thus enabling them to move from pond to pond or from stream to stream. In cold climates the fishes must either migrate to warmer latitudes in winter, as some do, or withstand variously the cold, often freezing weather. Some fish can be frozen solid, and yet thaw out and resume active living. Some lie at the bottoms of deep pools through the colder periods, while many others, such as the minnows, chubs, and other kinds common in small streams, bury themselves in the mud, and lie dormant or asleep through the whole winter. On the other hand in countries where the long intense rainless summers dry up the pools, some fishes have the habit of burying themselves in the mud, which, with slime from the body, forms about them a sort of tight cement ball in which they lie dormant until the rains come. "Thus a lung-fish (called Protopterus), found in Asia and Africa, so completely slimes a ball of mud around it that it may live for more than one season, perhaps many; it has been dug up and sent to England, still enclosed in its round mud-case, and when it was placed in warm water it awoke as well as ever."
Food-fishes and fish-hatcheries.—Most fishes are suitable for food, though not all. Some are too small to be worth catching or too bony to be worth eating. Some of the larger ones, especially the sharks, are tough and rank. A few are bitter and in the tropics a number of species feed on poisonous coelenterates about the coral reefs, becoming themselves poisonous in turn. But a fish is rarely poisonous or unwholesome unless it takes poisonous food. Where fishes of a kind specially used for food gather in great numbers at certain seasons of the year, fishing is carried on extensively and with an elaborate equipment. Such fisheries, some of which have been long known, are scattered all over the world. Along the shores of the Mediterranean Sea, and on the coasts of Norway, France, the British Isles and Japan are numerous great fishing-places. But "nowhere are there found such large fisheries as those along the northern Atlantic coasts of our own continent, extending from Massachusetts to Labrador. Especially on the banks of Newfoundland are codfish, herring, and mackerel caught." Among our fresh-water fisheries the great salmon fisheries of the Penobscot and Columbia rivers and of the Karluk and other rivers of Alaska are the best known. The whitefish of our Great Lakes is also one of the important food-fishes of the world.
In many places fishes are raised in so-called hatcheries, not usually for immediate consumption but for the purpose of stocking ponds and streams either in the neighborhood of the hatchery or in distant waters which the special species cultivated has not been able naturally to reach. The eggs of some fishes are large and non-adherent, two features which greatly favor artificial impregnation and hatching. In the hatcheries the eggs are put first into warm water, where development begins; they are then removed into cool water, which arrests development without injury, making shipment possible. The eggs of salmon and trout in particular can be sent long distances to suitable streams or ponds. The eggs of the shad have been thus carried from the East to the streams of California and trout have been distributed to many streams in our country which by themselves they could never have reached.
The salmon is a conspicuous example of those fishes which can be artificially propagated. The eggs of the salmon are large, firm, and separate from each other. If the female fish be caught when the eggs are ripe and her body be pressed over a pan of water the eggs will flow out into the water. By a similar process the milt or male sperm-cells can be procured and poured over the eggs to fertilize them. The young after hatching are kept for a few days or weeks in artificial pools, till the yolk-sacs are absorbed and they can take care of themselves. They are then turned into the stream, where they drift tail foremost with the current and pass downward to the sea. All trout may be treated in similar fashion, but there are many food-fishes which cannot be handled in this way. In some the eggs are small or soft, or viscid and adhering in bunches. In others the life-habits make artificial fertilization impossible. Such species are artificially reared only by catching the young and taking them from one stream to another. To this type belong the black bass, the sunfish, the catfish and other familiar forms.
CHAPTER XXV
BRANCH CHORDATA (Continued). CLASS BATRACHIA: THE BATRACHIANS
The structure, life-history, and habits of the garden-toad (Bufo lentiginosus) have already been studied (see Chapter II and Chapter XII).
OTHER BATRACHIANS.
The class Batrachia includes the animals familiarly known as cœcilians, sirens, mud-puppies, salamanders, toads, and frogs. Although differing plainly from fishes in appearance and habits, the batrachians are really closely related to them, resembling them in all but a few essential characters. Among the distinctive characters of batrachians may be noted the absence of fins supported by fin-rays, the presence usually of well-developed legs for walking or leaping, and the absence or reduction of certain bones of the head connected with the gills and lower jaw and which are well developed in the fishes. The batrachians stand in somewhat intermediate position between the fishes and the reptiles, showing some of the characters of both. They are, like fishes and reptiles, cold-blooded. In their adult condition some are terrestrial and some aquatic as to habitat, but all have an aquatic larval life. The water-inhabiting young breathe at first by means of gills, later lungs begin to develop, and for a time both gills and lungs are used in respiration. Finally in the adult condition in almost all of the forms the gills are wholly lost and breathing is done by the lungs and skin solely. Correlated with the change of habits from larval to adult stage there is usually a well-marked metamorphosis in post-embryonic development. This metamorphosis is specially striking among the frogs and toads. None of the aquatic forms is marine, salt water always killing eggs, larvæ or adults. Batrachians are found all over the world, although there are few in the extreme North. They are most abundant in warm and tropical lands.
Body form and organization.—The body varies from a long and slender, truly snake-like form as in the tropical cœcilians through the usual salamander (fig. 119) shape, where it is more robust but still elongate and tailed, to the heavy, squat, tailless condition of the toads. Legs, with five digits, are usually present, and are used for swimming, walking, or leaping. The legs are longest and best developed in the short tailless frog and toad forms which are mostly terrestrial, and are short and weak in the tailed salamander forms, many of which are aquatic. The skin is almost always naked, showing a marked difference from the scaled condition of reptiles and most of the fishes, and its cells secrete a slimy, sticky, usually whitish fluid, which in some cases is irritating, or even poisonous. The skin is sometimes thrown up into folds or ridges, and in some species is elevated to form a kind of fin on the tail or back. This unpaired fin differs from the dorsal fin (and other fins) of fishes in not being supported by rayed processes of the skeleton. There are in some batrachians traces of an exoskeleton in the presence of scale-like structures in the skin or in the horny nails on the digits, but these cases are rare. The skin contains pigment-cells and many of the batrachians are brilliantly colored and patterned; some of the pigment is carried by special contractile or expansile cells, the chromatophores (see account of chromatophores of the Cephalopoda, p. 256), so that the animal can change its tint and markings more or less rapidly. All the batrachians possess external gills in their aquatic larval stage, and in a few forms, as the sirens and mud-puppies, gills are retained all through life. These gills are branched folds of the skin abundantly supplied with blood-vessels.
In the organization of the batrachian body the usual vertebrate characters appear, the body-organs being arranged with reference to a supporting and protecting internal bony skeleton. The head is plainly set off from the rest of the body and bears the mouth and the organs of hearing and sight. Certain so-called lateral sense organs, the function of which is not exactly known, occur arranged in three lines on each side of the body of some of the forms. Both pairs of limbs are present and functional in almost all of the species. In the cœcilians the limbs are wholly wanting; in the sirens only the fore legs are present.
Structure.—The most obvious skeletal differences among batrachians are those due to variations in external form. While there are as many as 100 vertebræ in some of the elongate long-tailed salamanders (even 250 in the strange snake-like cœcilians), there are but 10 (the last or tenth being the rod-shaped bone called the urostyle) in the short, tailless frogs and toads. To any of the vertebræ except the first (the single cervical vertebra) and the last, ribs may be attached and the cœcilians have about as many pairs of ribs as vertebræ. In the frogs and toads, however, the ribs are lost. In any case they are never fastened by their lower ends to the breast-bone.
The alimentary canal is usually not much longer than the body and is plainly divided into mouth, pharynx, œsophagus, small intestine, large intestine or rectum, and anal opening. The teeth when present occur on both the jaws and the palate. They are small, sharp, point backward and are fused to the bones. They are wholly wanting in the toad and in some other allied forms. The tongue may be wanting, or may be immovably fixed to the floor of the mouth, or as in the frogs, fastened at its front end but free behind, so that the hinder end can be protruded far from the mouth for the purpose of catching insects.
The organs of respiration are gills, external and internal, lungs, trachea or windpipe, and the skin. In the earliest larval stages all batrachians have gills; later, in most cases, the gills become reduced and disappear, while at the same time lungs are developing. In some salamanders the lungs never develop, but the animals, in their adult stage, breathe wholly by means of the skin. In a few cases, as in the siren and mud-puppies, gills are retained through the whole life, although lungs are also present in the adult stage. The lungs are two in number, a right and a left lung, and are simple sacs with the walls more or less folded or thrown into ridges and richly supplied with blood-vessels. The front end of the lungs opens directly into the pharynx or, in the more elongate batrachians, is connected with it by a tubular trachea or windpipe. In the frogs and toads there are vocal cords stretched across the short windpipe; the vibration of these cords produces the croaking.
The heart is always three-chambered, consisting of the right and left auricles and a single ventricle. The circulation of the more generalized salamanders like the mud-puppies is essentially like that of a fish. In the frogs and toads there is a distinct advance beyond this condition. The red corpuscles of the blood are oval in shape and are the largest found among any of the vertebrates.
In the nervous system the small size of the hindbrain or cerebellum is noticeable. The sense organs are fairly well developed. The skin of the whole body is provided with tactile nerve-endings. There are special taste organs on the lining membrane of the tongue and mouth-cavity. The eyes have no lids in some of the lower forms; most of the frogs and toads have an upper lid but no under one, although a thin membrane, called the nictitating membrane, arises from the lower margin of the eye and can be drawn up over it. The ears have no external parts, other than the thin tympanic membranes. The nostrils of frogs and toads can be closed by the contraction of certain special muscles.
Life-history and habits.—The sexes are distinct, and in most cases the young hatch from eggs. A few of the salamanders give birth to free young. The eggs are usually in strings or chains enclosed in a clear gelatinous substance; these chains of eggs are either simply dropped into the water or are fastened to water-plants. The young, called tadpoles (fig. 120), in their earlier larval stages are extremely fish-like in character, long-bodied, tailed, swimming freely about by means of the fin-like flattened tail, and breathing by means of external gills. Nor do they show any sign of legs. As the tadpoles grow and develop the legs begin to appear, the hind legs first in the frogs and toads, the fore legs first in the salamanders; lungs develop and the gills disappear (except in the cases of the few forms which retain gills through life). The tail shortens and finally disappears in the frogs and toads; with the salamanders the tail-fin only is lost. At the same time the change from water to land is made. Further growth is very slow; frogs are not really adult, that is, capable of producing young, until they are five years old, and they may continue to increase in size until they are ten years old.
The food of the adult batrachians is almost exclusively small animals, particularly insects and worms. Crustaceans, snails, and young fish are also eaten. The tadpoles also eat vegetable matter. Almost all batrachians are nocturnal in habit, remaining concealed by day. In the zones in which cold winters occur they hibernate or pass the winter in a torpid condition, or state of "suspended animation," or, as it is said, they sleep through the winter. Frogs burrow into the mud at the bottom of ponds at the approach of winter and come forth early in the spring to lay their eggs. Most batrachians are very tenacious of life, being able to withstand long periods of fasting and serious mutilation, and most of them can regenerate certain lost parts, such as the tail or legs.
Classification.—The living Batrachia are divided into three orders, viz., the Urodela, including the sirens, mud-puppies, salamanders, and newts, batrachians which retain the tail throughout life, having generally two pairs of limbs of approximately equal size, and sometimes possessing gills or gill-slits in the adult condition; the Anura, or frogs and toads, with no tail in the adult condition, with short and broad trunk, with hind limbs greatly exceeding the fore limbs in size, and never with gills or gill-slits in the adult stage; and the Gymnophiona, or cæcilians, snake-like batrachians having neither limbs nor tail, with a dermal exoskeleton and without gills or gill-slits in the adult.
Mud-puppies, salamanders, etc. (Urodela).—Technical Note.—If possible obtain specimens of mud-eels (Siren), common in the South, or mud-puppies (Necturus), common in the central North, as examples of batrachians with gills persisting in the adult stage. One or more species of Amblystoma may be found in almost any part of the country, and larvæ of large size may be found with the external gills. For an example of the general long-tailed or Urodelous type of batrachian any salamander or newt occurring in the vicinity of the school may be used. The little green triton or eft (Diemictylus viridiscens) of the eastern States, or its larger brown-backed congener of the Pacific coast (D. torosus) is common in water, while another eft, the little red-backed salamander, (Plethodon) is common in the woods under logs and stones. The external characters of the body should be compared with those of the toad. The skeleton should be prepared by macerating away the flesh (for directions, see p. 452), and the presence of the many caudal vertebræ and the ribs, the equality in size of the legs, and other points should be noted. Compare with skeleton of toad. Make drawings. It will be well, also, to dissect out and examine the various internal organs of the salamander, comparing them with the same organs in the toad. The salamander, indeed, is in many ways better than the toad as an example of the class. Its body is less adaptively modified and shows the essentially fish-like character of the batrachian structure.
The batrachians which retain external gills in the adult stage are the members of two families of which the American representatives are known as mud-eels (Siren) and mud-puppies or water-dogs (Necturus). The mud-eels, which are found "in the ditches in the swamps of the southern States from South Carolina to the Rio Grande of Texas and up the Mississippi as high as Alton, Illinois," are blackish in color, have no hind legs and are long and slender, with the tail shorter than the rest of the body. They reach a length of nearly three feet. The mud-puppies, found in the Great Lakes and in the rivers of the upper Mississippi valley, are brown with colored spots, and are about two feet long when full grown. They have both fore and hind legs.
A few salamanders, while not possessing external gills when adult, have a spiracle or small circular opening in the side of the neck which leads into the throat. The best-known American salamander of this kind is the large heavy-bodied blackish water-dog or "hellbender" (Cryptobranchus) of the Ohio River. It is about two feet long, and is "a very unprepossessing but harmless creature." It has a conspicuous longitudinal fold of skin along each side of the body. The largest known batrachian, the giant salamander of Japan (Megalobatrachus), reaching a length of three feet, is related to the water-dog.
Of all the salamanders the most interesting are the blunt-nosed salamanders (Amblystoma). A dozen or more species of Amblystoma occur in North America, of which tigrinum, a dark-brown species with many irregular yellow blotches sometimes arranged in cross-bands, is the most widespread. The larvæ of some Amblystoma retain their gills until they have reached a large size, and in one or two species the usual metamorphosis is very long delayed and the salamanders produce young while in the larval condition, that is, while retaining the gills and a compressed fin-like tail. In the case of a certain Mexican species (A. maculatum) it is believed that the final metamorphosis never occurs. The Mexicans call these gilled larval Amblystoma axolotls, and use them for food. For a long time naturalists supposed the Amblystoma larvæ which produce young to be the adults of a species of salamanders which retained their gills through life, like the sirens and mud-puppies, and classified them in a distinct genus.
Of the various common salamanders or newts some are found in streams, ponds, and ditches, and some under logs and stones in the woods. The aquatic forms have the tail compressed (flattened from side to side), while the land forms have the tail cylindrical, tapering to a point. Most of the land-salamanders produce their young alive, while the water forms lay eggs which are usually attached to a submerged plant-stem. The salamanders are, almost without exception, found only in the northern hemisphere.
Frogs and toads (Anura).—There are about a dozen species of frogs in the United States. The largest of these, and indeed the largest of all the frogs, is the well-known bullfrog (Rana catesbiana), which reaches a length (head to posterior end of body) of eight inches. It is found in ponds and sluggish streams all over eastern United States and in the Mississippi valley. It is greenish in color with the head usually bright pale green. Its croaking is very deep and sonorous. The pickerel-frog (R. palustris), which is bright brown on the back with two rows of large oblong square blotches of dark brown on the back, is found in the mountains of eastern United States. The little pale reddish-brown wood-frog (R. sylvatica) with arms and legs barred above is common in damp woods and is "an almost silent frog." The peculiar and infrequently seen frogs known as the "spade-foots" (Scaphiopus) are subterranean in habit and usually live in dry fields or even on arid plains and deserts. They pass through their development and metamorphosis very rapidly, appearing immediately after a rain and laying their eggs in temporary pools. At this time of egg-laying they utter extraordinarily loud and strange cries. Some frogs in other parts of the world live in trees, and the eggs of one species are deposited on the leaves of trees, leaves which overhang the water being selected so that the issuing young may drop into it.
The true tree-frogs or tree-toads (Hylidæ) constitute a family especially well represented in tropical America. They have little disk- or pad-like swellings on the tips of their toes to enable them to hold firmly to the branches of the trees in which they live. Some, like the swamp tree-frog and the cricket-frog, are not arboreal in habit, remaining almost always on the ground. The common tree-frog of the eastern States (Hyla versicolor) is green, gray, or brown above with irregular dark blotches, and yellow below. It croaks or trills, especially at evening and in damp weather. Pickering's tree-frog (Hyla pickeringii) makes the "first note of spring" in the eastern States. This tree-frog is the one most frequently heard in the autumn too, but "its voice is less vivacious than in the spring and its lonely pipe in dry woodlands is always associated with goldenrods and asters and falling leaves." The tree-frogs of North America lay their eggs in the water on some fixed object as an aquatic plant, in smaller packets than those of the true frogs, and not in strings as do the toads.
The toads (Bufonidæ) differ from the true frogs in having no teeth and in not having, as the frogs do, a cartilaginous process uniting the shoulder-bones of the two sides of the body. The absence of this uniting process makes the thoracic region capable of great expansion. There are only a few species of toads in North America, but one of these species, the common American toad (Bufo lentiginosus), is very abundant and widespread. It appears also in two or three varieties, the common toad of the southern States differing in several particulars from that of the northern. The toad is a familiar inhabitant of gardens, and does much good by feeding on noxious insects. It is most active at twilight. Its eggs are laid in a single line in the centre of a long slender gelatinous string or rope, which is nearly always tangled and wound round some water-plant or stick near the shore on the bottom of a pond. The eggs are jet black and when freshly laid are nearly spherical. At the time of egg-laying the toads croak or call, making a sort of whistling sound and at the same time pronouncing deep in the throat "bu-rr-r-r-r." The toad does not open its mouth when croaking, but expands a large sac or resonator in its throat. The toad-tadpoles are blacker than those of frogs or salamanders, and undergo their metamorphosis while of smaller size than those of frogs. When they leave the water they travel for long distances, hopping along so vigorously that in a few days they may be as far as a mile from the pond where they were hatched. They conceal themselves by day, but will appear after a warm shower; this sudden appearance of many small toads sometimes gives rise to the false notion that they have fallen with the rain.
Cœcilians (Gymnophiona).—The third order of batrachians, the cœcilians, includes about twenty species of slender worm- or snake-like limbless forms which are confined to the tropics. Some of them are wholly blind and the others have only rudimentary eyes. In them the skin is folded at regular intervals so that the body appears to be rigid or segmented, and in some species there are small concealed horny scales in the skin.
CHAPTER XXVI
BRANCH CHORDATA (Continued). CLASS REPTILIA: THE SNAKES, LIZARDS, TURTLES, CROCODILES, ETC.
THE GARTER SNAKE (Thamnophis sp.)
Technical Note.—Garter snakes may be found almost anywhere during the spring and summer months. If possible each student should have a specimen, but in case it is difficult to get enough snakes two students can use a single specimen. If garter snakes are rare, take any other snake. Snakes will live a long time without feeding and specimens should be kept alive until ready to use. Kill with chloroform as directed for the toad (p. 5). After completing the study of the external characters place each specimen in a dissecting-pan and with a pair of scissors cut through the scales on the ventral side, passing backwards from the eighteenth to the fortieth. Pin back the edges of the cut and thus expose the heart. Through its lower end, the ventricle, insert a large canula; inject with a fairly large syringe the glue mass which is described on p. 452. This injection will fill the entire arterial system. To inject the venous system make another cut through the ventral scales, cutting forward from the anal scale through about forty of them. Note the injected mass in some of the vessels already filled. Take one of the large vessels still containing blood and pass two ligatures beneath it. Get ready a small canula and cut a slit in the vessel, elevating the head so that the blood will run out as much as possible. Now wash the blood off, insert the canula in the slit and tie one ligature about the vessel containing the canula; have the other ready to tie after the vein has been injected. Use a new color for the venous system. Leave specimen in cold water for a time until the injection is hard. Then continue the cut from the anal plate forward to the lower jaw and pin out the edges of the cut on both sides in the dissecting-pan.
Structure (fig. 122).—Note that the snake is covered with horny scales somewhat as the fish is. How do these scales differ from those of the fish? In snakes the scales are not bony, but are true skin structures. Note the modification of the scales on the head, back, and ventral surface. Those on the dorsal surface often have minute ridges, the keels. How do the ventral scales differ from the dorsal ones and others? By a system of muscles these ventral scales are rhythmically moved and as their posterior edges are pushed back against some resisting object the body glides forward. On the head note the pair of eyes. Are there eyelids? In front of each eye note an opening. What are these openings? Thrust a bristle into the opening and see where it enters the mouth-cavity through the internal nares. Does the snake have external ears? Observe the very long jaws and note that they are loosely hinged. Examine the inside of the mouth. Are there teeth? If so where are they situated, and how arranged? Note that all of the teeth point backwards. Food is not chewed. When some object of prey, a frog, or mouse, for example, is seized, the teeth hold it fast to the roof of the mouth and by a backward and forward movement of the lower jaws it is gradually drawn into the large œsophagus. What is the character and situation of the tongue? Just behind the tongue note the narrow slit, glottis, opening into the windpipe, or trachea. Back of the trachea opens the œsophagus.
When the snake is laid open the elongate heart will be conspicuous in the anterior third of the body. Insert a blowpipe or quill into the glottis just back of the tongue, and inflate the lung, which is a long, thin-walled bag extending from the region of the heart posteriorly for two-thirds of the length of the body. There is but one developed lung, the right; note at the anterior end of the lung a small mass of tissue, the atrophied left lung. Running forward from the lung is a long tube composed of incomplete cartilaginous rings, connected by membrane, the trachea. Note the long straight alimentary canal. Distinguish the œsophagus, stomach, intestine, rectum and the anus.
In the region of the lung is an elongated dark-red glandular mass, the liver. The secretion from the liver passes down through the long hepatic duct to the oval-shaped green gall-bladder and into the intestine.
Technical Note.—The bile-duct may be injected through the gall-bladder with some colored injecting mass.
Note that the duct running off from the gall-bladder to the intestine passes through a pink glandular organ, the pancreas. At the anterior end of the pancreas is a dark-red nodular structure, the spleen. The alimentary canal, the liver and the spleen are all suspended from the dorsal wall of the body-cavity by a delicate sheet of tissue. What is this? This condition we have also noted in the toad and fish.
Toward the posterior end of the body cavity are two long, dark-red glands, the kidneys, which are the principal excretory organs of the body. Through a long, slender tube (the ureter) each of the kidneys passes off its wastes. Where do the ureters open?
Anterior to the kidneys are the reproductive organs. The eggs, produced by the female snake, after being fertilized, pass backward through the egg-tubes. During the breeding season these tubes are much distended. This is due to the presence of the developing eggs, for the young snakes are hatched in the egg-tubes.
A successful injection as directed in the first technical note will have filled both arterial and venous systems. How does the general shape of the snake's heart compare with that of the toad? The heart consists of two ventricles, incompletely separated, and two auricles. In the snake the conus arteriosus is very much shortened and is not visible. Note two large vessels arising from the median portion of the ventricle. The one on the left side is the left aortic artery or left aortic arch, while the right gives off two branches. Where does the anterior one of these run? The main branch, or right aortic arch, passes back to meet its fellow, the left aortic artery, forming with it the dorsal aorta, which runs posteriorly to the end of the tail. Note the various branches given off by the dorsal aorta and trace some of them. Arising from the ventricles beneath the two aortic arches is the pulmonary artery, which goes to the lung. There the blood is purified, after which it is taken up by the pulmonary vein and carried back to the left auricle, whence it passes into the ventricle to be mixed with the impure blood from the right auricle. From the arteries the blood flows to all parts of the body through fine capillaries, bathing the tissues, giving off oxygen and taking up the carbonic acid gas. From these capillaries it passes into veins and so back to the heart; from the anterior end of the body through the jugular veins and from the posterior portion of the body through the postcaval vein. Flowing forward from the tail in the caudal vein, the blood enters the capillaries of the kidneys, where the waste matter is taken from it. This part of the circulatory system is known as the renal-portal circulation. From the kidneys the blood flows through the postcaval vein anteriorly to the heart.
The blood which passes out from the dorsal aorta to all parts of the alimentary canal is again collected into veins which unite to form the mesenteric vein. This vein runs to the liver, where it breaks up into capillaries. Thence the blood is carried into the postcaval vein, which leads directly to the heart. This part of the circulatory system which collects blood from the alimentary canal and carries it to the liver is called the hepatic-portal system.
Just in front of the heart will be noted a nodular structure, the thyroid gland, while a little in advance of the thyroid may be seen a long glandular mass, the thymus gland. The functions of these glands are not certainly understood.
Remove the alimentary canal and muscles from a part of the body and note that the axial skeleton, like that of the other vertebrates studied, consists of a series of vertebræ placed end to end. Are there arms or legs? Are shoulder and pelvic girdles present? How many of the vertebræ bear ribs? The ribs connect at their lower ends with the ventral scales. Note the great number of the vertebræ and ribs as compared with those of the toad or fish. What are those vertebræ called which bear no appendages or ribs? Examine carefully the elongated skull of the snake, especially the modified jaws. A detailed study of the skeleton may be made by referring to the account of the skeleton of the lizard in Parker's "Zootomy," pp. 130 et seq.
The nervous system may be worked out in a specimen which has been immersed in 20 per cent nitric acid. The description of the nervous system of the toad (see pp. 12-13) will suffice for a guide to the study of the nervous system of the snake. The special sense organs, as eyes and ears, should be examined and compared with those of the fish and toad.
Life-history and habits.—The garter snakes are more or less aquatic in habit and are good swimmers. They are often found far from water, but in greatest abundance where the cat-tails and rushes grow thickest. They feed on frogs, salamanders, and field-mice, which they swallow whole. All the garter snakes are ovoviviparous, i.e., hatch eggs within the body-cavity. The eggs, often as many as eighteen or twenty, are enclosed within widened portions of the oviducts during embryonic existence; when the young are born they are able to shift for themselves. During cold weather the garter snake hibernates, hiding then in some gopher-hole, or, in the warmer climates, under some log or stone, there to lie dormant until the warm days of spring come, when it resumes activity.
The garter snake sheds its skin at least once a year, sometimes oftener. This process may be observed in snakes kept in confinement. For some time before molting the animal remains torpid, the eyes become milky, and the skin loses its lustre. After a few days it conceals itself, the skin about the lips and snout pulls away and the animal slips out of its entire skin. The snake not only sheds the skin of the body but also the covering of the eyes. Snakes have no eyelids, as we have already noted, that which represents the eyelid being a transparent membrane which covers the eyeball.
No species of the garter snake group is poisonous. Sometimes a garter snake may appear to be vicious, but its teeth are very short and at best it can only make a small scratch scarcely piercing the skin.