Elementary Zoology Part 21

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 Hydrophidae 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 america.n.u.s_), living in the West Indies and South America and occasionally found in Florida, and the American alligator (_Alligator mississippiensis_), common in the mora.s.ses 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.

FOOTNOTE:

[17] By many zoologists the lizards and snakes are held to form two distinct orders, Lacertilia and Ophidia.

CHAPTER XXVII

BRANCH CHORDATA (_Continued_). CLa.s.s AVES: THE BIRDS

THE ENGLISH SPARROW (_Pa.s.ser 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 cla.s.s 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 h.o.r.n.y substance. This h.o.r.n.y substance together with the feathers and h.o.r.n.y 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 _nict.i.tating 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 ca.n.a.l 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 _vibrissae_ about the nostrils.

In the determination of birds by the use of a cla.s.sificatory "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."

TECHNICAL NOTE.--Pull the feathers from the body, being careful not to tear the skin.

In the fish and toad, already studied, the head is closely joined to the trunk. How is it with the bird? Observe that the _knee_ of the sparrow is covered by feathers and that it is the _ankle_ which extends down as the bare unfeathered part to the _digits_. How many digits have the feet of the bird? How are they arranged?

=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.

[Ill.u.s.tration: FIG. 133.--Diagrammatic outline of bird's body with names of external parts and regions.]

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 _sophagus_. Inflate the sophagus with a blowpipe and note how distensible is its lower end near the breast. This distensible portion is called the _crop_. If the alimentary ca.n.a.l be drawn out straight the sophagus 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 sophagus 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 _caeca_. The short part of intestine between the caeca and cloaca is called the _r.e.c.t.u.m_. 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 ca.n.a.l, cutting it free posteriorly at the caeca 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 ma.s.s 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 pa.s.sing 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 pa.s.s 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 praecavae_ and the _postcava_. Each praecava 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 cla.s.s-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 s.p.a.ces 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 ma.s.s; 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 pa.s.s 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 alb.u.minous portion, together with several enveloping membranes and the hard sh.e.l.l 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 princ.i.p.al _cranial nerves_. Work out the _spinal cord_ and princ.i.p.al _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).

[Ill.u.s.tration: FIG. 132.--Dissection of the English sparrow, _Pa.s.ser domesticus_.]

In the sparrow's _skeleton_ note the compactness of the _skull_ and the fusion of its bones. Observe the long _cervical vertebrae_ which support the skull, also the _thoracic vertebrae_ bearing the ribs and sternum. How many of each of these kinds of vertebrae are there? The vertebrae 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 vertebrae, 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 a.s.sumed 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 cla.s.ses 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 _pterylae_, separated by bare or downy s.p.a.ces 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 vibrissae 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 ca.s.sowaries 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 h.o.r.n.y sheath. In some water and sh.o.r.e 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-s.p.a.ces 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 ca.n.a.l, 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 a.s.sist in the grinding. The lungs of birds are more complex than those of batrachians and reptiles, being divided into small s.p.a.ces by numerous membranous part.i.tions. 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-s.p.a.ces in the hollow bones. By this arrangement the bird can fill with air not only its lungs but all the special air-sacs and s.p.a.ces 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 pa.s.sing 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-sh.e.l.l with the "egg-tooth,"

a h.o.r.n.y 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).

[Ill.u.s.tration: FIG. 134.--The nest and eggs of the black phbe, _Sayornis nigricans_. (Photograph by J. O. Snyder.)]

More details regarding the eggs, nest, and young of birds will be given later in this chapter.

=Cla.s.sification.=--The cla.s.s Aves is usually divided into numerous orders, the number and limits of these as published in zoological manuals varying according to the opinions of various zoologists. The rank of an order in this group is far lower than in most other cla.s.ses. In other words, the orders are very much alike and are recognized mainly for the convenience in breaking up the vast a.s.semblage of species. In North America practically all the ornithologists have agreed upon a scheme of cla.s.sification, which will therefore be adopted in this book. According to this cla.s.sification the eight hundred (approximately) known species of North American birds represent seventeen orders. Certain recognized orders, for example, the ostriches, are not represented naturally in North America at all. As birds can usually be readily identified, the species being easily distinguished by general external appearance, and as there are many excellent book-guides to their cla.s.sification, the beginning student can specially well begin with them his study of systematic zoology, which concerns the identification and cla.s.sification of species. In a later paragraph are given therefore some suggestions for field and laboratory work in the determination of local bird-faunae. In the following paragraphs each of the American orders is briefly discussed, as is also the foreign order of ostriches.