FOOTNOTE:
[16] The author wishes to call the attention of teacher and student to the plan (referred to in the Preface, page v) adopted in writing the directions for the dissections. The sequence of the references to the various organs depends on the actual course of the dissection, and not upon the a.s.sociation of organs in systems. And the directions are so much condensed that they are hardly more than a means of orienting the student, leaving him to work out independently, or by the aid of more detailed accounts (sometimes specifically referred to), the details of the dissection.
CHAPTER XXV
BRANCH CHORDATA (_Continued_). CLa.s.s 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 cla.s.s Batrachia includes the animals familiarly known as ccilians, 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, larvae 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.
[Ill.u.s.tration: FIG. 119.--The tiger salamander. (From Jenkins and Kellogg.)]
=Body form and organization.=--The body varies from a long and slender, truly snake-like form as in the tropical ccilians 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 h.o.r.n.y 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 chromatoph.o.r.es (see account of chromatoph.o.r.es 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 ccilians 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 vertebrae in some of the elongate long-tailed salamanders (even 250 in the strange snake-like ccilians), 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 vertebrae except the first (the single cervical vertebra) and the last, ribs may be attached and the ccilians have about as many pairs of ribs as vertebrae. 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 ca.n.a.l is usually not much longer than the body and is plainly divided into mouth, pharynx, sophagus, small intestine, large intestine or r.e.c.t.u.m, and a.n.a.l 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 nict.i.tating 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 s.e.xes 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.
[Ill.u.s.tration: FIG. 120.--Tadpoles. (Photograph from life by Cherry Kearton; permission of Ca.s.sel & Co.)]
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 pa.s.s 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.
=Cla.s.sification.=--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 caecilians, 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 larvae 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 vertebrae 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 cla.s.s. 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 "h.e.l.lbender" (_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 j.a.pan (_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 larvae 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_ larvae 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 cla.s.sified them in a distinct genus.
[Ill.u.s.tration: FIG. 121.--The Western brown eft, or salamander, _Diemyctylus torosus_. (From living specimen.)]
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. pal.u.s.tris_), 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 pa.s.s 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 (Hylidae) const.i.tute 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 a.s.sociated 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 (Bufonidae) 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 sh.o.r.e 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 p.r.o.nouncing 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.
=Ccilians (Gymnophiona).=--The third order of batrachians, the ccilians, 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 h.o.r.n.y scales in the skin.
CHAPTER XXVI
BRANCH CHORDATA (_Continued_). CLa.s.s 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, pa.s.sing 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 ma.s.s 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 a.n.a.l scale through about forty of them.
Note the injected ma.s.s in some of the vessels already filled. Take one of the large vessels still containing blood and pa.s.s 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 a.n.a.l 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 h.o.r.n.y _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 sophagus. 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 _sophagus_.
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 ma.s.s 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 ca.n.a.l_. Distinguish the _sophagus_, _stomach_, _intestine_, _r.e.c.t.u.m_ and the _a.n.u.s_.
In the region of the lung is an elongated dark-red glandular ma.s.s, the _liver_. The secretion from the liver pa.s.ses 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 ma.s.s.
Note that the duct running off from the gall-bladder to the intestine pa.s.ses through a pink glandular organ, the _pancreas_. At the anterior end of the pancreas is a dark-red nodular structure, the _spleen_. The alimentary ca.n.a.l, 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 princ.i.p.al excretory organs of the body. Through a long, slender tube (the _ureter_) each of the kidneys pa.s.ses 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, pa.s.s 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.