Elementary Zoology, Second Edition

The class Reptilia includes the lizards, snakes, tortoises, turtles, crocodiles, and alligators. Although popularly associated in the common mind with the batrachians, the reptiles are really more nearly related to the birds than to the salamanders and frogs. In general shape they more nearly resemble the batrachians, but in the structural condition of the internal body organs they are more like the birds. They are cold-blooded, and breathe exclusively by means of lungs, the forms which live in water coming to the surface to breathe. They are covered with horny scales or plates, which with the entire absence of gills after hatching readily distinguish them from all the batrachians. While most reptiles live on land, some inhabit fresh water and some the ocean. As the young have the same habitat and general habits as the adult, there is no such metamorphosis in their life-history as is shown by the batrachians. The reptiles are widespread geographically, occurring, however, in greatest abundance in tropical regions, and being wholly absent from the Arctic zone. They are not capable of such migrations as are accomplished by the birds and many mammals, but withstand severely hot or cold seasons by passing into a state of suspended animation or seasonal sleep or torpor.

Body form and organization.—The chief variations in body form among the reptiles are manifest when a turtle, lizard, and snake are compared. In the turtles, the body is short, flattened, and heavy, and provided always with four limbs, each terminating in a five-toed foot; in the lizards the body is more elongate and with usually four legs, but sometimes with two only, or even none at all; while in the snakes the long, slender, cylindrical body is legless or at most has mere rudiments of the hinder limbs. With the reptiles locomotion is as often effected by the bending or serpentine movements of the trunk as by the use of legs. Among lizards and snakes the body is covered with horny epidermal scales or plates, while among the turtles and crocodiles there may be, in addition to the epidermal plates, a real deposit of bone in the skin whereby the effectiveness of the armor is increased. The epidermal covering of snakes and lizards is periodically molted, or, as we say, the skin is shed. The bright colors and patterns of snakes and of many lizards are due to the presence and arrangement of pigment-cells in the skin. Among some reptiles, notably the chameleons, the colors and markings can be quickly and radically changed by an automatic change in the tension of the skin.

Structure.—In reptiles, as in batrachians, the chief variations in the body skeleton are correlated with differences in external body form. In the short compact body of the turtles and tortoises the number of vertebræ is much smaller than in the snakes. Some turtles have only 34 vertebræ; certain snakes as many as 400. The reptilian skull, in the number and disposition of its parts and in the manner of its attachment to the spinal column, resembles that of the birds, although the cranial bones remain separate, not fusing as in the birds. In the snake the two halves of the lower jaw are not fused in front but are united by elastic ligaments, which condition, together with the extremely mobile articulation of the base of the jaws, allows the snakes to open their mouths so as to take in bodies of great size. All of the reptiles, except the turtles, are provided with small teeth which serve, generally, for seizing or holding prey and not for mastication. The poisonous snakes have one or more long, sharp, and grooved or hollow fangs (fig. 131). In the legless reptiles both shoulder and pelvic girdles may be wholly lacking; in the limbed forms both girdles are more or less well developed.

The tongue of many reptiles, notably the snakes, is bifid or forked, and is an extremely mobile and sensitive organ. The œsophagus is long and in the snakes can be stretched very wide so as to permit the swallowing of large animals whole. Reptiles breathe solely by lungs, of which there is a pair. They are simple and sac-like, the left lung being often much smaller than the other. In turtles and crocodiles the lungs are divided internally by septa into a number of chambers. Because of the rigidity of the carapace or "box" of turtles the air cannot be taken in the ordinary way by the use of the ribs and rib-muscles, but has to be swallowed. The reptilian heart consists of two distinct auricles and of two ventricles, which in most reptiles are only incompletely divided, the division into right and left ventricles being complete only among the crocodiles and alligators, the most highly organized of living reptiles.

The organs of the nervous system reach a considerable degree of development in the animals of this class. The brain in size and complexity is plainly superior to the batrachian brain and resembles quite closely that of birds. Of the organs of special sense those of touch are limited to special papillæ in the skin of certain snakes and many lizards. Taste seems to be little developed, but olfactory organs of considerable complexity are present in most forms, and consist of a pair of nostrils with olfactory papillæ on their inner surfaces. The ears vary much in degree of organization, crocodiles and alligators being the only reptiles with a well-defined outer ear. This consists of a dermal flap covering a tympanum. Eyes are always present and are highly developed. They resemble the eyes of birds in many particulars. All reptiles, excepting the snakes and a few lizards, have movable eyelids, including a nictitating membrane like that of the birds. With the snakes the eye is protected by the outer skin, which remains intact over it, but is transparent and thickened to form a lens just over the inner eye. Turtles and lizards have a ring of bony plates surrounding the eyes similar to that of the birds. In addition to the usual eyes there is in many lizards a remarkable eye-like organ, the so-called pineal eye, which is situated in the roof of the cranium, and is believed to be the vestige of a true third eye, which in ancient reptiles was probably a well-developed organ.

Life-history and habits.—Most reptiles lay eggs from which the young hatch after a longer or shorter period of incubation. Usually the eggs are simply dropped on the ground in suitable places (although certain turtles dig holes in which to deposit them), where they are incubated by the general warmth of the air and ground. However, some of the giant snakes, the pythons for instance, hold the eggs in the folds of the body. In the case of some snakes and lizards the eggs are retained in the body of the mother until the young hatch; such reptiles are said to be ovoviviparous, because the young, although born alive, are in reality enclosed in an egg until the moment of birth. Among reptiles the newly hatched young resemble the parents in most respects except in size, yet striking differences in coloration and pattern are not rare. But there is in this class no metamorphosis such as characterizes the post-embryonic development of the batrachians.

The food of reptiles consists almost exclusively of animal substance, although some species, notably the green turtles and certain land-tortoises, are vegetable-feeders. The animal-feeders are mostly predaceous, the smaller species catching worms and insects, while the larger forms capture fishes, frogs, birds, and their eggs, small mammals, and other reptiles.

Classification.—The living Reptilia are divided into four orders, of which one includes only a single genus, Hatteria, a peculiar lizard found in New Zealand. The other three are the Squamata, which includes the lizards and snakes,[17] distinguished by the scaly covering of the body, the Chelonia, which includes the tortoises and turtles, distinguished by the shell of bony plates which encloses the body, and the Crocodilia, which includes the crocodiles and alligators, whose bodies are covered with rows of sculptured bony scutes.

Tortoises and turtles (Chelonia).—Technical Note.—Obtain specimens of some pond- or land-turtle common in the vicinity of the school. The red-bellied and yellow-bellied terrapins (Pseudemys) or the painted or mud-turtles (Chrysemys) are common over most of the United States. (Pseudemys is found south of the Ohio River and Chrysemys north of it.) They may be raked up from creek-bottoms or fished for with strong hook and line, using meat as bait. They will live through the winter if kept in a cool place, without food or special care of any kind. Observe their swimming and diving, the retraction of head and limbs into the shell, the use of the third eyelid (nictitating membrane), and the swallowing of air.

Examine the external structure of a dead specimen (kill by thrusting a bit of cotton soaked with chloroform or ether into the windpipe; see opening just at base of tongue). Note shell consisting of a dorsal plate, the carapace, and ventral plate, the plastron, and the lateral uniting parts, the bridge. Note legs, and head with horny beak but no teeth. Compare with snake. The examination of the internal structure of the turtle can be readily made by sawing through the bridge on either side and removing the plastron. Note the ligaments which attach the plastron to the shoulder and pelvic girdles. Note muscles covering these bones. Note just behind the shoulder girdle the heart (perhaps still pulsating) and the dark liver on each side of it. Work out the alimentary canal, the trachea and lungs, and other principal organs, comparing them with those of the snake. The skeleton can be studied by dissecting and boiling and brushing away the flesh which still adheres to the bones. The comparison of the skeleton of the turtle with that of the snake is very instructive; marked differences in the skeletons of the two kinds of reptiles are obviously correlated with the differences in habits and shape of body. Note in the skeleton of the turtle especially the shoulder and pelvic girdles and limbs (absent in the snake) and small number of vertebræ and ribs.

Among the common turtles and tortoises of the United States are several species of soft-shelled turtles (Trionychidæ) with carapace not completely ossified and both carapace and plastron covered by a thick leathery skin which is flexible at the margins; the snapping-turtle (Chelydra serpentina), common in streams and ponds, with shell high in front and low behind and head and tail long and not capable of being withdrawn into the shell; the red-bellied and yellow-bellied terrapins (Pseudemys), red and yellow, with greenish-brown and black markings, common on the ground in woods and among rocks and also near water and sometimes in it; the pond- or mud-turtle (Chrysemys), also brightly colored and usually confined to ponds and pond-shores; and the box-tortoise (Cistudo carolina), common in woods and upland pastures and readily recognizable by its ability to enclose itself completely in its shell by the closing down of the lids of the plastron. All of these fresh-water and land-turtles except the soft-shelled turtles belong to one family, the Emydidæ, but have somewhat diverse habits. Most of them are carnivorous, but few catch any very active prey. While some are strictly aquatic, others are as strictly terrestrial, never entering the water. The eggs of all are oblong and are deposited in hollows, sometimes covered in sand. The newly hatched young are usually circular in shape, and vary in color and pattern from the parents.

The "diamond-back terrapin" (Malaclemmys palustris), used for food, is a salt-water form "inhabiting the marshes along the Atlantic coast from Massachusetts to Texas. About Charleston [and Baltimore] they are very abundant and are captured in large numbers for market, especially at the breeding season, when the females are full of eggs. Further north they are dug from the salt mud early in their hibernation and are greatly esteemed, being fat and savory."

Strongly contrasting with the usually small land- and fresh-water turtles are the great sea-turtles, such as the leather-back, the loggerhead and the green turtles. Some of these animals reach a length of six feet and more and a weight of nine hundred pounds, and have the feet compressed and fin-shaped for swimming. They live in the open ocean, coming on land only to lay their eggs, which are buried in the sand of ocean islands. These egg-laying visits are almost always made at night, and the turtles are then often caught by "turtlers." The flesh of most of the sea-turtles is used for food, and from the shell of certain species, notably the "hawk-bill" (Eretmochelys imbricata) the beautiful "tortoise-shell" used for making combs and other articles is obtained. The common green turtle (Chelonia mydas) of the Atlantic coast is the species most prized for food. It is a vegetarian, feeding on the roots of Zostera, the plant known in New England as eel-grass, though farther south it is called turtle-grass. When grazing the turtles eat only the roots, the tops thus rising to the surface, where they indicate to the turtler the animal's whereabouts. The turtler, armed with a strong steel barb attached to a rope and loosely fitted to the end of a pole, carefully rows up to the unsuspecting animal, and with a strong thrust plunges the barb through its shell, withdraws the pole, and, grasping the rope, now firmly attached to the turtle's back, lifts the animal to the surface. Here, with assistance, he turns it into the boat, where it is rendered helpless by being thrown on its back and by having its flippers tied. These turtles are also caught on their breeding-grounds, being found on the sand at night by the turtler, turned over on their backs, and left thus securely caught until assistance comes to help get them into the boats.

Snakes and lizards (Squamata).—Technical Note.—A snake has already been dissected and studied. It will be instructive to compare the external structures, at least, of a lizard with that of the snake. Specimens of some species of the common swift (Sceloporus) are obtainable almost anywhere in the United States. The "pine-lizards" of the east belong to this genus. Lizards may be sought for in woods, along fences, and especially on warm rocks.

The group of lizards is a very large one, about 1,500 species being known, but it is represented in the United States by comparatively few species. Lizards are especially abundant in the tropics of South America. The strange and fantastic appearance presented by some of them has made certain species the object of much interest and often fear on the part of the natives of tropical lands. In those regions are current extraordinary stories and beliefs regarding the habits and attributes of certain lizards like the basilisk and chameleon. Lizards are all more or less elongate and some are truly snake-like in form. The legs, though usually present and functional, are in many cases much reduced, and in some forms, as the glass-snake, either one or both pairs are so rudimentary as to have no external projection whatever. Although lizards are often regarded as being poisonous, only one genus, Heloderma, the Gila Monster, is really so. All others are perfectly harmless as far as poison is concerned, and most of them are unusually timid. They vary in size from a few inches to six feet in length. Most of them are terrestrial, some arboreal, and some aquatic.

Among the lizards of this country the swifts and ground-lizards are familiar everywhere. In certain regions the glass-snake or joint-snake (Opheosaurus ventralis) is common. This animal, popularly considered to be a snake, has no external limbs, and its tail is so brittle, the vertebræ composing it being very fragile, that part of it may break off at the slightest blow. In time a new tail is regenerated. It lives in the central and northern part of the United States, and burrows in dry places. In the western part of the country horned toads (Phrynosoma) are common, about ten different species being known. These are lizards with shortened and depressed body and well-developed legs. The body is covered with protective spiny protuberances, and in individual color and pattern resembles closely the soil, rocks, and cactus among which the particular horned toad lives. All the species of Phrynosoma are viviparous, seven or eight young being born alive at a time.

In New Mexico, Arizona, and northern Mexico the only existing poisonous lizards, the Gila Monster (Heloderma) (fig. 126) is found. This is a heavy, deep-black, orange-mottled lizard about sixteen inches long. There is much variance of belief among people regarding the Gila Monster, but recent experiments have proved the poisonous nature of the animal. The poison which is secreted by glands in the lower jaw flows along the grooved teeth into the wound. A beautiful and interesting little lizard found in the South is the green chameleon (Anolis principalis). Its body is about three inches long with a slender tail of five or six inches. The normal color of the chameleon is grass-green, but it may "assume almost instantly shades varying from a beautiful emerald to a dark and iridescent bronze color."

In the tropics many of the lizards reach great size and are of strange shape and patterns. The flying dragons (Draco) have a sort of parachute on each side of the body composed of a fold of skin supported by five or six false posterior ribs. These lizards live in the trees of the East Indies and "fly" or sail from tree to tree. They are very beautifully colored. The iguanas (Iguana) of the tropics of South America are commonly used for food. They live mostly in trees, and reach a length of five or six feet. The monitor (Varanus niloticus) is a great water-lizard that lives in the Nile, and feeds on crocodiles' eggs, of which it destroys great numbers. It is the principal enemy of the crocodile. When full grown it reaches a length of six feet or even more.

About 1,000 living species of snakes are known. Usually they have the body regularly cylindrical, and without distinct division into body regions. Legs are wanting, locomotion being effected by the help of the scales and the ribs. No snake can move forward on a perfectly smooth surface and no snake can leap. In some forms, such as the pythons, external rudiments of the hind limbs are present, but do not aid in locomotion. The mouth is large and distensible so that prey of considerably greater size than the normal diameter of the snake's body is frequently swallowed whole. The sense of taste is very little if at all developed, as the food is swallowed without mastication. The tongue, which is protrusible and usually red or blue-black, serves as a special organ of touch. Hearing is poor, the ears being very little developed. The sense of sight is also probably not at all keen. Snakes rely chiefly on the sense of smell for finding their prey and their mates. The colors of snakes are often brilliant, and in many cases serve to produce an effective protective resemblance by harmonizing with the usual surroundings of the animal. The food of snakes consists almost exclusively of other animals, which are caught alive. Some of the poisonous snakes kill their prey before swallowing it, as do some of the constrictors. While most snakes live on the ground, some are semi-arboreal and others spend part or all of their time in water. Cold-region snakes spend the winter in a state of suspended animation; in the tropics, on the contrary, the hottest part of the year is spent by some species in a similar "sleep."

There are so many common snakes in the United States that only a few of the more familiar forms can be mentioned. The non-poisonous species of America belong to the family Colubridæ, while all but one of the poisonous species belong to the family Crotalidæ, characterized by the presence of a pair of erectile poison-fangs on the upper jaw. Among the commonest of the Colubridæ are the garter snakes (Thamnophis) (fig. 127), always striped and not more than three feet long. The most widespread species is Thamnophis sirtalis, rather dully colored with three series of small dark spots along each side. The common water-snake (Natrix sipedon) is brownish with back and sides each with a series of about 80 large square dark blotches alternating with each other. It feeds on fishes and frogs, and although "unpleasant and ill-tempered" is harmless. One of the prettiest and most gentle of snakes is the familiar little greensnake (Cyclophis æstivus), common in the East and South in moist meadows and in bushes near the water. It feeds on insects and can be easily kept alive in confinement. A familiar larger snake is the blacksnake or blue racer (Bascaniom constrictor), "lustrous pitch black, general color greenish below and with white throat." It is "often found in the neighborhood of water, and is particularly partial to thickets of alders, where it can hunt for toads, mice, and birds, and being an excellent climber it is often seen among the branches of small trees and bushes, hunting for young birds in the nest." The chain-snake (Lampropeltis getulus) of the southeast and the king-snake (also a Lampropeltis) (fig. 128) of the central States are beautiful lustrous black-and-yellow spotted snakes which feed not only on lizards, salamanders, small birds and mice but also on other snakes. The king-snake should be protected in regions infested by "rattlers." The spreading adder or blowing viper (Heterodon platirhinos), a common snake in the eastern States, brownish or reddish with dark dorsal and lateral blotches, depresses and expands the head when angry, hissing and threatening. Despite the popular belief in its poisonous nature this ugly reptile is quite harmless. It specially infests dry sandy places.

With the exception of the coral or beadsnake (Elaps fulvius), a rather small jet-black snake with seventeen broad yellow-bordered crimson rings, found in the southern States, the only poisonous snakes of the United States are the rattlesnakes and their immediate relatives, the copperhead and water-moccasin. These snakes all have a large triangular head, and the posterior tip of the body is, in the rattlesnakes, provided with a "rattle" composed of a series of partly overlapping thin horny capsules or cones of shape as shown in figure 130. These horny pieces are simply the somewhat modified successively formed epidermal coverings of the tip of the body, which instead of being entirely molted as the rest of the skin is, are, because of their peculiar shape, loosely attached to one another, and by the basal one to the body of the snake. The number of rattles does not correspond to the snake's years for several reasons, partly because more than one rattle can be added to the tail in a year, and especially because rattles are easily and often broken off. As many as thirty rattles have been found on one snake. There are two species of ground-rattlesnakes or massasaugas (Sistrurus) in the United States and ten species of the true rattlesnakes (Crotalus). The centre of distribution of the rattlesnakes is the dry tablelands of the southwest in New Mexico, Arizona, and Texas. But there are few localities in the United States outside the high mountains in which "rattlers" do not occur or did not occur before they were exterminated by man. The copperhead (Agkistrodon contortix) is light chestnut in color, with inverted Y-shaped darker blotches on the sides, and seldom exceeds three feet in length. It occurs in the eastern and middle United States from Pennsylvania and Nebraska southward. It is a vicious and dangerous snake, striking without warning. The water-moccasin (Agkistrodon piscivorous) is dark chestnut-brown with darker markings. The head is purplish black above. It is found along the Atlantic and Gulf coasts from North Carolina to Mexico, extending also some distance up the Mississippi valley. It is distinctively a water-snake, being found in damp swampy places or actually in water. It reaches a length of over four feet and is a very venomous snake, striking on the slightest provocation. The common harmless water-snake is often called water-moccasin in the southern States, being popularly confounded with this most dangerous of our serpents. The poison of all of these snakes is a rather yellowish, transparent, sticky fluid secreted by glands in the head, from which it flows through the hollow maxillary fangs. The character and position of the fangs are shown in figure 131. Remedial measures for the bite of poisonous snakes are, first, to stop, if possible, the flow of blood from the wound to the heart, by compressing the veins between the wound and heart, then to suck (if the lips are unbroken) the poison from the wound, next to introduce by hypodermic injection permanganate of potash, bichloride of mercury or chromic acid into the wound, and finally perhaps to take some strong stimulant as brandy or whiskey.

Of the kinds of snakes not found in this country perhaps the most interesting are the gigantic boa constrictors, anacondas, and pythons. Pythons are found in India, the islands of the Malay archipelago, and Australia, while the boas and anacondas live in the tropics of America. The largest pythons reach a length of thirty feet and some of the boas are nearly as large. These snakes feed on small mammals such as fawns, kids, water-rats, etc., and birds. The prey is swallowed whole, being first encircled and crushed to death in folds of the body. After a meal the python or boa lies in a sort of torpor for some time. A famous snake is the deadly cobra-da-capello of India. These snakes are so abundant and the bite is so nearly certainly fatal that thousands of persons are killed each year in India by it. Other extremely poisonous snakes are the vipers (Vipera cerastes), which live in the hot deserts of northern Africa. Over each eye there is a scaly spine or horn, from which the name horned viper is derived. The most poisonous snake of South Africa is the large and ugly puff-adder, which puffs itself up when irritated. An interesting group of snakes is that of the Hydrophidæ or sea-snakes, which swim on the surface of the ocean by means of their flattened and oar-like tails. These forms live in the tropical portions of the Indian and Pacific oceans, ranging as far north as the Gulf of California, and spend their whole life in the water, "out of which they appear to be blind and soon die." They are extremely venomous, but are all of small size, rarely two feet long.

Crocodiles and alligators (Crocodilia).—The crocodiles and alligators are reptiles familiar by name and appearance, though seen in nature only by inhabitants or visitors in tropical and semitropical lands. In the United States there are two species of these great reptiles, the American crocodile (Crocodilus americanus), living in the West Indies and South America and occasionally found in Florida, and the American alligator (Alligator mississippiensis), common in the morasses and stagnant pools of the southern States. The alligator differs from the crocodiles in having a broader snout. It is rarely more than twelve feet long. The best-known crocodile is the Nile crocodile, which is not limited to the Nile, but is found throughout Africa. In the Ganges of India is found another member of this group of reptiles called the gavial. It is among the largest of the order, reaching a length of twenty feet. The crocodiles, alligators, and gavials comprise not more than a score of species altogether, but because of their wide distribution, great size, and carnivorous habits they are among the most conspicuous of the larger living animals. They live mostly in the water, going on land to sun themselves or to lay their eggs. They move very quickly and swiftly in water but are awkward on land. Fish, aquatic mammals and other animals which occasionally visit the water are their prey. The gavial and Nile crocodile are both known to attack and devour human beings, and these species annually cause a considerable loss of life. But few such fatalities, however, are accredited to the American alligator.

CHAPTER XXVII

BRANCH CHORDATA (Continued). CLASS AVES: THE BIRDS

THE ENGLISH SPARROW (Passer domesticus)

Technical Note.—The English sparrow may be found now in cities and villages all over the United States. It has become a veritable pest, and the killing of the few needed for the laboratory may be looked on as desirable rather than deplorable, as is the killing of birds in almost all other cases. The males have a black throat, with the other head-markings strong and contrasting (black, brown, and white), while the females have a uniform grayish and brownish coloration on the head.

Specimens are best taken alive, as shooting usually injures them for dissection. One can rely on the ingenuity of the boys of the class to procure a sufficient number of specimens. Observations on the habits of the birds should be made by the pupils as they go to and from school. For dissection use fresh specimens if possible. If desirable a pigeon or dove may be used in place of the sparrow.

External structure.—Note in the sparrow the same general arrangement of body parts as in the toad, the body being divided into head, upper limbs, trunk, and lower limbs. In the toad, however, all of the limbs are fitted for walking and jumping, whereas in the sparrow the anterior pair of appendages, the wings, are modified to be organs of flight, and the posterior limbs are specially adapted for perching. Note that the sparrow is covered with feathers, some long, some short, in some places thick and in others thin, but all fitting together to form a complete covering for the body. Note also that the anterior end of the head is prolonged into a hard bony structure, the bill, covered with horny substance. This horny substance together with the feathers and horny covering of the feet are modified portions of the skin. Note the long quill-feathers attached to the posterior edge of the wing. By these the bird sustains its flight. Other long quill-feathers are attached to the posterior end of the body, forming the tail. By a system of muscles connected with these feathers they act together, serving as a rudder during flight and as a balancing contrivance when perching. Note just above the bill two openings protected by tufts of feathers. What are these openings? How are they connected with the mouth? Note the large eyes, and at the inner angle of each the delicate nictitating membrane which can be drawn over the ball. Does the bird have external ears? Lift the feathers just above the tail (the upper tail-coverts) and note a small median gland, the oil-gland, from which the bird derives the oil with which it oils its feathers. Beneath the tail note the opening from the alimentary canal and from the kidneys and reproductive organs. This is called the cloacal opening.

Examine in detail some of the feathers. In one of the quill-feathers note the central stem or shaft composed of two parts, a basal hollow quill, which bears no web and by which the feather is inserted in the skin, and a longer, terminal, four-sided portion, the rachis, which bears on either side a web or vane. Each vane is composed of many narrow linear plates, the barbs, from which rise (like miniature vanes) many barbules. Each barbule bears many fine barbicels and hamuli or hooklets. The barbs of the feather are interlocked. How is this effected? The feathers which overlie the whole body and bear the color pattern are called contour-feathers. How do they differ from or correspond with the quill-feathers in structure? Soft feathers called down-feathers or plumules, cover the body more or less completely, being, however, mostly hidden by the contour-feathers; the barbs of these are sometimes not borne on a rachis, but arise as a tuft from the end of the quill. Certain other feathers which have an extremely slender stem and usually no vane, except a small terminal tuft of barbs, are called thread-feathers, or filoplumules. They are rather long, but are mostly hidden by the contour-feathers. In certain birds they stand out conspicuously, as the vibrissæ about the nostrils.

In the determination of birds by the use of a classificatory "key" (see p. 359) it is necessary to be familiar with the names applied to the various external regions of the body and plumage, and with the terms used to denote the special varying conditions of these parts. By reference to figure 133 the names of the regions or parts most commonly referred to may be learned. A full account of all of the external characters with definitions of the various terms used in referring to them may be found in Coues's "Key to North American Birds."

Internal structure (fig. 132).—Technical Note.—With a pair of scissors cut just beneath the skin anteriorly from the cloacal opening to the angle of the lower jaw. Pin the sparrow on its back by the wings, feet, and bill. Push back the skin from both sides and pin out.

Note the large powerful pectoral muscles. Note a hard median projection of bone, the sternum, which is a large keel-shaped bone with lateral expansions to which are attached the ribs. Where are the largest and most powerful muscles of the toad located? Where are they in the fish? In the bird the most powerful muscles are these pectoral muscles, which move the wings in flight.

Technical Note.—Cut the pectoral muscles from the left side of the sternum, push back and pin to one side. With a strong pair of scissors cut through the ribs on the left side of the sternum and through the overlying bones. Lift the whole sternum, with the right pectoral muscle attached, to the left side of the pan and pin it down. Cut through the membrane which covers the viscera and cover the dissection with water.

In this operation note the V-shaped wishbone in front of the sternum. It is composed of the two clavicles with their inner ends fused. Note the stout coracoid bones extending from the anterior end of the sternum to the shoulder.

Note near the middle of the body the heart with the large blood-vessels proceeding from it. Behind the heart lies the large reddish-brown liver, and on the left side below the liver is the large gizzard or muscular stomach. Note the viscera folded over themselves in the body-cavity. Push them temporarily aside and note in the dorsal region under the heart large pinkish spongy sacs, the lungs. These are attached by short tubes, the bronchi, to the long cartilaginous trachea. At the union of the bronchi with the trachea is a small expansion with cartilaginous walls, within which are stretched small bands of muscles. This organ is the syrinx, the song- or voice-apparatus of the bird. It should be cut open and carefully examined. Trace the trachea forward to its anterior end. It opens by a glottis into the larynx, a slightly swollen chamber with cartilaginous walls. Note the U-shaped hyoid bone surrounding the front of the glottis. Through a blowpipe or quill inserted into the glottis blow air into the trachea and observe the inflation of the lungs and of certain large air-sacs in the abdomen, which communicate with them.

Beneath the trachea note the long œsophagus. Inflate the œsophagus with a blowpipe and note how distensible is its lower end near the breast. This distensible portion is called the crop. If the alimentary canal be drawn out straight the œsophagus will be found to run as an almost straight tube down the left side of the body to the gizzard. This latter organ has very thick muscular walls and in it the food is ground up among the small bits of gravel it contains. Extending from the gizzard near the entrance of the œsophagus note the long pyloric loop of the intestine called duodenum. Within this loop is a long pinkish gland, the pancreas, which empties by a duct into the duodenum. Into the duodenum also the overlying liver empties its secretion of bile from the median-placed gall-bladder. From the duodenum the small intestine or ileum extends with many convolutions to its exit through the cloacal aperture. On the intestine near the cloacal opening note a pair of glandular structures, the cæca. The short part of intestine between the cæca and cloaca is called the rectum. On the left side of the body beneath the gizzard note a dark glandular structure, the spleen.

Make a drawing of the dissection as so far worked out.

Technical Note.—Remove the alimentary canal, cutting it free posteriorly at the cæca and anteriorly just above the muscular gizzard. Cut open the gizzard and note its structure. The contained sand and gravel grains are picked up by the bird as it eats.

On either side of the throat note the well-defined thyroid gland; in young sparrows will be noted on each side of the neck a mass of tissue, the remains of the thymus gland, which disappears in the adult.

Cut transversely through the lower end of the heart and note that the ventricles are wholly distinct, whereas in the toad and snake they are incompletely separated. In the bird there is a complete double circulation. Its blood is not mixed, the pure with the impure, as in the toad and snake. Blood passing through the right auricle and ventricle goes to the lungs; on its return to the heart purified, it enters the left auricle and left ventricle thence to pass out over the body through the arteries.

Note the large aorta given off from the left ventricle. Note the two large branches, the innominate arteries, given off by it near its origin. Each innominate divides into three smaller arteries, a carotid, branchial, and pectoral. The aorta itself turns toward the back and continues posteriorly through the body as the dorsal aorta. To the right auricle come three large veins, the right and left præcavæ and the postcava. Each præcava is formed by three veins, the jugular from the head, the branchial from the wing, and the pectoral from the pectoral muscles. The postcava comes from the liver. From the right ventricle go the short right and left pulmonary arteries to the lungs, and from the lungs the blood is brought to the left auricle through the right and left pulmonary veins.

Technical Note.—For a detailed study of the circulation of the bird the teacher should inject the blood system of some larger bird, as a pigeon or fowl, for a class-demonstration. (For a guide, use Parker's "Zootomy," p. 209, or Martin and Moale's "How to Dissect a Bird," pp. 135-140 and pp. 148, 149.)

In the posterior dorsal region of the body-cavity will be found large three-lobed organs fitting into the spaces between the bones of the back on either side. These are the kidneys, and from their outer margins on each side a ureter runs posteriorly into the cloaca. Overlying the anterior ends of the kidneys are the reproductive organs. In the male these glands consist of firm, whitish, glandular bodies. From each runs a long convoluted vas deferens, which enters the cloaca. This tube corresponds to the egg-duct of the female. In the female the right egg-gland and egg-duct or oviduct are wanting. The left egg-gland appears as a glandular mass; during the breeding season yellow ova or eggs in various stages of development project from its surface. The oviduct opens by a funnel-shaped mouth near the egg-gland and runs thence to the cloaca. The eggs pass from the egg-gland into the body-cavity, where they are caught in the upper end of the oviduct and carried down and out through the cloacal opening. It is in the oviduct that the egg derives its accessory covering, which consists of a white or albuminous portion, together with several enveloping membranes and the hard shell enclosing all.

Remove the top of the skull and note the large brain. What portions of the brain make up the greater part of it? Note the differences between this brain and that of the toad. Trace the principal cranial nerves. Work out the spinal cord and principal spinal nerves. For an account of the nervous system of the sparrow see Martin and Moale's "How to Dissect a Bird," pp. 150-163.

Technical Note.—For a study of the skeleton of the sparrow a specimen should be cleaned by boiling in a soap-solution (see p. 452).

In the sparrow's skeleton note the compactness of the skull and the fusion of its bones. Observe the long cervical vertebræ which support the skull, also the thoracic vertebræ bearing the ribs and sternum. How many of each of these kinds of vertebræ are there? The vertebræ posterior to the thorax are more or less fused together to form the sacrum, which, with the pelvic girdle, supports the leg-bones. The bones of the tail consist of a number of very small vertebræ, some of which are fused together. Note the correspondence between the bones of the leg and those of the wing. What are the names of each of the bones of each limb, and what are the corresponding bones in the two limbs? The wings and legs being modified for different uses, their various bones have assumed different relations to each other and to the body, for they are bent at directly opposite angles and the attachment of muscles is different. Compare the skeleton of the bird with that of the toad. (For a detailed account of the skeleton of the bird see Parker's "Zootomy," pp. 182-209, or Martin and Moale's "How to Dissect a Bird," pp. 102-125.)

Life-history and habits.—The English sparrow was first introduced into the United States in 1850, and since that time has rapidly populated most of the cities and towns of the country. On account of its extreme adaptability to surroundings, its omnivorous food-habits and its fecundity it survives where other birds would die out. It also crowds out and has caused the disappearance or death of other birds more attractive and more useful. The sparrow annually rears five or six broods of young, laying from six to ten eggs at each sitting. Had it no enemies a single pair of sparrows would multiply to a most astonishing number. The sparrow has, however, a number of enemies, most common among them perhaps being the "small boy," but birds and mammals play the chief part in the destruction. The smaller hawks prey upon them, and rats and mice destroy great numbers of their young and of their eggs whenever the nests can be reached. The sparrow is omnivorous and when driven to it is a loathsome scavenger, though at other times its tastes are for dainty fruits. Its senses of perception are of the keenest; it can determine friend or foe at long range. The nesting habits are simple, the nests being roughly made of any sort of twigs and stems mixed with hair and feathers and placed in cornices or trees. A maple-tree in a small Missouri town contained at one time thirty-seven of these nests.

OTHER BIRDS.

Birds are readily and unmistakably distinguishable from all other kinds of animals by their feathers. They are further distinguished from the reptiles on one hand by their possession of a complete double circulation and by their warm blood (normally of a temperature of from 100-112° F.), and from the mammals on the other by the absence of milk-glands. There are about 10,000 known species of living birds; they occur in all countries, being most numerous and varied in the tropics. Birds are exceptionally available animals for the special attention of beginning students, because of their abundance and conspicuousness and the readiness with which their varied and interesting habits may be observed. The bright colors and characteristic manners which make the identification of the different kinds easy, the songs and flight, and the feeding, nesting and general domestic habits of birds are all excellent subjects for personal field-studies by the students. We shall therefore devote more attention to the birds than to the other classes of vertebrates, just as we selected the insects among the invertebrates for special consideration.

Body form and structure.—The general body form and external appearance of a bird are too familiar to need description. The covering of feathers, the modification of the fore limbs into wings, and the toothless, beaked mouth are characteristic and distinguishing external features. The feathers, although covering the whole of the surface of the body, are not uniformly distributed, but are grouped in tracts called pterylæ, separated by bare or downy spaces called apteria. They are of several kinds, the short soft plumules or down-feathers, the large stiffer contour-feathers, whose ends form the outermost covering of the body, the quill-feathers of the wings and tail, and the fine bristles or vibrissæ about the eyes and nostrils called thread-feathers. The fore limbs are modified to serve as wings, which are well developed in almost all birds. However, the strange Kiwi or Apteryx of New Zealand with hair-like feathers is almost wingless, and the penguins have the wings so reduced as to be incapable of flight, but serving as flippers to aid in swimming underneath the water. The ostriches and cassowaries also have only rudimentary wings and are not able to fly. Legs are present and functional in all birds, varying in relative length, shape of feet, etc., to suit the special perching, running, wading, or swimming habits of the various kinds. Living birds are toothless, although certain extinct forms, known through fossils, had large teeth set in sockets on both jaws. The place of teeth is taken, as far as may be, by the bill or beak formed of the two jaws, projecting forward and tapering more or less abruptly to a point. In most birds the jaws or mandibles are covered by a horny sheath. In some water and shore forms the mandibular covering is soft and leathery. The range in size of birds is indicated by comparing a humming-bird with an ostrich.

Many of the bones of birds are hollow and contain air. The air-spaces in them connect with air-sacs in the body, which connect in turn with the lungs. Thus a bird's body contains a large amount of air, a condition helpful of course in flight. The breast-bone is usually provided with a marked ridge or keel for the attachment of the large and powerful muscles that move the wings, but in those birds like the ostriches, which do not fly and have only rudimentary wings, this keel is greatly reduced or wholly wanting. The fore limbs or wings are terminated by three "fingers" only; the legs have usually four, although a few birds have only three toes and the ostriches but two.

As birds have no teeth with which to masticate their food, a special region of the alimentary canal, the gizzard, is provided with strong muscles and a hard and rough inner surface by means of which the food is crushed. Seed-eating birds have the gizzard especially well developed, and some birds take small stones into the gizzard to assist in the grinding. The lungs of birds are more complex than those of batrachians and reptiles, being divided into small spaces by numerous membranous partitions. They are not lobed as in mammals, and do not lie free in the body-cavity, but are fixed to the inner dorsal region of the body. Connected with the lungs are the air-sacs already referred to, which are in turn connected with the air-spaces in the hollow bones. By this arrangement the bird can fill with air not only its lungs but all the special air-sacs and spaces and thus greatly lower its specific gravity. The vocal utterances of birds are produced by the vocal cords of the syrinx or lower larynx, situated at the lower end of the trachea just where it divides into the two bronchial tubes, the tracheal rings being here modified so as to produce a voice-box containing two vocal cords controlled by five or six pairs of muscles. The air passing through the voice-box strikes against the vocal cords, the tension of which can be varied by the muscles. In mammals the voice-organ is at the upper or throat end of the trachea.

The heart of birds is composed of four distinct chambers, the septum between the two ventricles, incomplete in the Reptilia, being in this group complete. There is thus no mixing of arterial and venous blood in the heart. The systemic blood-circulation being completely separated from the pulmonic, the circulation is said to be double. The circulation of birds is active and intense; they have the hottest blood and the quickest pulse of all animals. In them the brain is compact and large, and more highly developed than in batrachians and reptiles, but the cerebrum has no convolutions as in the mammals. Of the special senses the organs of touch and taste are apparently not keen; those of smell, hearing, and sight are well developed. The optic lobes of the brain are of great size, relatively, compared with those of other vertebrate brains, and there is no doubt that the sight of birds is keen and effective. The power of accommodation or of quickly changing the focus of the eye is highly perfected. The structure of the ear is comparatively simple, there being ordinarily no external ear, other than a simple opening. The organs of the inner ear, however, are well developed, and birds undoubtedly have excellent hearing. The nostrils open upon the beak, and the nasal chambers are not at all complex, the smelling surface being not very extensive. It is probable that the sense of smell is not, as a rule, especially keen.

Development and life-history.—All birds are hatched from eggs, which undergo a longer or shorter period of incubation outside the body of the mother, and which are, in most cases, laid in a nest and incubated by the parents. The eggs are fertilized within the body of the female, the mating time of most birds being in the spring or early summer. Some kinds, the English sparrow, for example, rear numerous broods each year, but most species have only one or at most two. The eggs vary greatly in size and color-markings, and in number from one, as with many of the Arctic ocean birds, to six or ten, as with most of the familiar song-birds, or from ten to twenty, as with some of the pheasants and grouse. The duration of incubation (outside the body) varies from ten to thirty days among the more familiar birds, to nearly fifty among the ostriches. The temperature necessary for incubation is about 40° C. (100° F.). Among polygamous birds (species in which a male mates with several or many females) the males take no part in the incubation and little or none in the care of the hatched young; among most monogamous birds, however, the male helps to build the nest, takes his turn at sitting on the eggs, and is active in bringing food for the young, and in defending them from enemies. The young, when ready to hatch, break the egg-shell with the "egg-tooth," a horny pointed projection on the upper mandible, and emerge either blind and almost naked, dependent upon the parents for food until able to fly (altricial young), or with eyes open and with body covered with down, and able in a few hours to feed themselves (precocial young).