BRANCH CHORDATA (_Continued_). CLASS BATRACHIA:
THE BATRACHIANS
The structure, life-history, and habits of the garden-toad (_Bufo
lentiginosus_) have already been studied (see Chapter II and Chapter
XII).
OTHER BATRACHIANS.
The class Batrachia includes the animals familiarly known as cœcilians,
sirens, mud-puppies, salamanders, toads, and frogs. Although differing
plainly from fishes in appearance and habits, the batrachians are really
closely related to them, resembling them in all but a few essential
characters. Among the distinctive characters of batrachians may be noted
the absence of fins supported by fin-rays, the presence usually of
well-developed legs for walking or leaping, and the absence or reduction
of certain bones of the head connected with the gills and lower jaw and
which are well developed in the fishes. The batrachians stand in
somewhat intermediate position between the fishes and the reptiles,
showing some of the characters of both. They are, like fishes and
reptiles, cold-blooded. In their adult condition some are terrestrial
and some aquatic as to habitat, but all have an aquatic larval life. The
water-inhabiting young breathe at first by means of gills, later lungs
begin to develop, and for a time both gills and lungs are used in
respiration. Finally in the adult condition in almost all of the forms
the gills are wholly lost and breathing is done by the lungs and skin
solely. Correlated with the change of habits from larval to adult stage
there is usually a well-marked metamorphosis in post-embryonic
development. This metamorphosis is specially striking among the frogs
and toads. None of the aquatic forms is marine, salt water always
killing eggs, larvæ or adults. Batrachians are found all over the world,
although there are few in the extreme North. They are most abundant in
warm and tropical lands.
[Illustration: 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 cœcilians through
the usual salamander (fig. 119) shape, where it is more robust but
still elongate and tailed, to the heavy, squat, tailless condition of
the toads. Legs, with five digits, are usually present, and are used
for swimming, walking, or leaping. The legs are longest and best
developed in the short tailless frog and toad forms which are mostly
terrestrial, and are short and weak in the tailed salamander forms,
many of which are aquatic. The skin is almost always naked, showing a
marked difference from the scaled condition of reptiles and most of
the fishes, and its cells secrete a slimy, sticky, usually whitish
fluid, which in some cases is irritating, or even poisonous. The skin
is sometimes thrown up into folds or ridges, and in some species is
elevated to form a kind of fin on the tail or back. This unpaired fin
differs from the dorsal fin (and other fins) of fishes in not being
supported by rayed processes of the skeleton. There are in some
batrachians traces of an exoskeleton in the presence of scale-like
structures in the skin or in the horny nails on the digits, but these
cases are rare. The skin contains pigment-cells and many of the
batrachians are brilliantly colored and patterned; some of the pigment
is carried by special contractile or expansile cells, the
chromatophores (see account of chromatophores of the Cephalopoda, p.
256), so that the animal can change its tint and markings more or less
rapidly. All the batrachians possess external gills in their aquatic
larval stage, and in a few forms, as the sirens and mud-puppies, gills
are retained all through life. These gills are branched folds of the
skin abundantly supplied with blood-vessels.
In the organization of the batrachian body the usual vertebrate
characters appear, the body-organs being arranged with reference to a
supporting and protecting internal bony skeleton. The head is plainly
set off from the rest of the body and bears the mouth and the organs of
hearing and sight. Certain so-called lateral sense organs, the function
of which is not exactly known, occur arranged in three lines on each
side of the body of some of the forms. Both pairs of limbs are present
and functional in almost all of the species. In the cœcilians the limbs
are wholly wanting; in the sirens only the fore legs are present.
=Structure.=--The most obvious skeletal differences among batrachians
are those due to variations in external form. While there are as many
as 100 vertebræ in some of the elongate long-tailed salamanders (even
250 in the strange snake-like cœcilians), there are but 10 (the last
or tenth being the rod-shaped bone called the urostyle) in the short,
tailless frogs and toads. To any of the vertebræ except the first (the
single cervical vertebra) and the last, ribs may be attached and the
cœcilians have about as many pairs of ribs as vertebræ. In the frogs
and toads, however, the ribs are lost. In any case they are never
fastened by their lower ends to the breast-bone.
The alimentary canal is usually not much longer than the body and is
plainly divided into mouth, pharynx, œsophagus, small intestine, large
intestine or rectum, and anal opening. The teeth when present occur on
both the jaws and the palate. They are small, sharp, point backward
and are fused to the bones. They are wholly wanting in the toad and in
some other allied forms. The tongue may be wanting, or may be
immovably fixed to the floor of the mouth, or as in the frogs,
fastened at its front end but free behind, so that the hinder end can
be protruded far from the mouth for the purpose of catching insects.
The organs of respiration are gills, external and internal, lungs,
trachea or windpipe, and the skin. In the earliest larval stages all
batrachians have gills; later, in most cases, the gills become reduced
and disappear, while at the same time lungs are developing. In some
salamanders the lungs never develop, but the animals, in their adult
stage, breathe wholly by means of the skin. In a few cases, as in the
siren and mud-puppies, gills are retained through the whole life,
although lungs are also present in the adult stage. The lungs are two
in number, a right and a left lung, and are simple sacs with the walls
more or less folded or thrown into ridges and richly supplied with
blood-vessels. The front end of the lungs opens directly into the
pharynx or, in the more elongate batrachians, is connected with it by
a tubular trachea or windpipe. In the frogs and toads there are vocal
cords stretched across the short windpipe; the vibration of these
cords produces the croaking.
The heart is always three-chambered, consisting of the right and left
auricles and a single ventricle. The circulation of the more
generalized salamanders like the mud-puppies is essentially like that
of a fish. In the frogs and toads there is a distinct advance beyond
this condition. The red corpuscles of the blood are oval in shape and
are the largest found among any of the vertebrates.
In the nervous system the small size of the hindbrain or cerebellum is
noticeable. The sense organs are fairly well developed. The skin of the
whole body is provided with tactile nerve-endings. There are special
taste organs on the lining membrane of the tongue and mouth-cavity. The
eyes have no lids in some of the lower forms; most of the frogs and
toads have an upper lid but no under one, although a thin membrane,
called the nictitating membrane, arises from the lower margin of the eye
and can be drawn up over it. The ears have no external parts, other than
the thin tympanic membranes. The nostrils of frogs and toads can be
closed by the contraction of certain special muscles.
=Life-history and habits.=--The sexes are distinct, and in most cases
the young hatch from eggs. A few of the salamanders give birth to free
young. The eggs are usually in strings or chains enclosed in a clear
gelatinous substance; these chains of eggs are either simply dropped
into the water or are fastened to water-plants. The young, called
tadpoles (fig. 120), in their earlier larval stages are extremely
fish-like in character, long-bodied, tailed, swimming freely about by
means of the fin-like flattened tail, and breathing by means of
external gills. Nor do they show any sign of legs. As the tadpoles
grow and develop the legs begin to appear, the hind legs first in the
frogs and toads, the fore legs first in the salamanders; lungs
develop and the gills disappear (except in the cases of the few forms
which retain gills through life). The tail shortens and finally
disappears in the frogs and toads; with the salamanders the tail-fin
only is lost. At the same time the change from water to land is made.
Further growth is very slow; frogs are not really adult, that is,
capable of producing young, until they are five years old, and they
may continue to increase in size until they are ten years old.
[Illustration: FIG. 120.--Tadpoles. (Photograph from life by Cherry
Kearton; permission of Cassel & 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 pass the winter in
a torpid condition, or state of "suspended animation," or, as it is
said, they sleep through the winter. Frogs burrow into the mud at the
bottom of ponds at the approach of winter and come forth early in the
spring to lay their eggs. Most batrachians are very tenacious of life,
being able to withstand long periods of fasting and serious
mutilation, and most of them can regenerate certain lost parts, such
as the tail or legs.
=Classification.=--The living Batrachia are divided into three orders,
viz., the Urodela, including the sirens, mud-puppies, salamanders, and
newts, batrachians which retain the tail throughout life, having
generally two pairs of limbs of approximately equal size, and
sometimes possessing gills or gill-slits in the adult condition; the
Anura, or frogs and toads, with no tail in the adult condition, with
short and broad trunk, with hind limbs greatly exceeding the fore
limbs in size, and never with gills or gill-slits in the adult stage;
and the Gymnophiona, or cæcilians, snake-like batrachians having
neither limbs nor tail, with a dermal exoskeleton and without gills or
gill-slits in the adult.
=Mud-puppies, salamanders, etc. (Urodela).=--TECHNICAL NOTE.--If
possible obtain specimens of mud-eels (_Siren_), common in the
South, or mud-puppies (_Necturus_), common in the central North,
as examples of batrachians with gills persisting in the adult
stage. One or more species of _Amblystoma_ may be found in almost
any part of the country, and larvæ of large size may be found with
the external gills. For an example of the general long-tailed or
Urodelous type of batrachian any salamander or newt occurring in
the vicinity of the school may be used. The little green triton or
eft (_Diemictylus viridiscens_) of the eastern States, or its
larger brown-backed congener of the Pacific coast (_D. torosus_)
is common in water, while another eft, the little red-backed
salamander, (_Plethodon_) is common in the woods under logs and
stones. The external characters of the body should be compared
with those of the toad. The skeleton should be prepared by
macerating away the flesh (for directions, see p. 452), and the
presence of the many caudal vertebræ and the ribs, the equality in
size of the legs, and other points should be noted. Compare with
skeleton of toad. Make drawings. It will be well, also, to dissect
out and examine the various internal organs of the salamander,
comparing them with the same organs in the toad. The salamander,
indeed, is in many ways better than the toad as an example of the
class. Its body is less adaptively modified and shows the
essentially fish-like character of the batrachian structure.
The batrachians which retain external gills in the adult stage are the
members of two families of which the American representatives are known
as mud-eels (_Siren_) and mud-puppies or water-dogs (_Necturus_). The
mud-eels, which are found "in the ditches in the swamps of the southern
States from South Carolina to the Rio Grande of Texas and up the
Mississippi as high as Alton, Illinois," are blackish in color, have no
hind legs and are long and slender, with the tail shorter than the rest
of the body. They reach a length of nearly three feet. The mud-puppies,
found in the Great Lakes and in the rivers of the upper Mississippi
valley, are brown with colored spots, and are about two feet long when
full grown. They have both fore and hind legs.
A few salamanders, while not possessing external gills when adult, have
a spiracle or small circular opening in the side of the neck which leads
into the throat. The best-known American salamander of this kind is the
large heavy-bodied blackish water-dog or "hellbender" (_Cryptobranchus_)
of the Ohio River. It is about two feet long, and is "a very
unprepossessing but harmless creature." It has a conspicuous
longitudinal fold of skin along each side of the body. The largest known
batrachian, the giant salamander of Japan (_Megalobatrachus_), reaching
a length of three feet, is related to the water-dog.
Of all the salamanders the most interesting are the blunt-nosed
salamanders (_Amblystoma_). A dozen or more species of _Amblystoma_
occur in North America, of which _tigrinum_, a dark-brown species with
many irregular yellow blotches sometimes arranged in cross-bands, is
the most widespread. The larvæ of some _Amblystoma_ retain their gills
until they have reached a large size, and in one or two species the
usual metamorphosis is very long delayed and the salamanders produce
young while in the larval condition, that is, while retaining the
gills and a compressed fin-like tail. In the case of a certain Mexican
species (_A. maculatum_) it is believed that the final metamorphosis
never occurs. The Mexicans call these gilled larval _Amblystoma_
axolotls, and use them for food. For a long time naturalists supposed
the _Amblystoma_ larvæ which produce young to be the adults of a
species of salamanders which retained their gills through life, like
the sirens and mud-puppies, and classified them in a distinct genus.
[Illustration: 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. palustris_), which is bright brown on the back with
two rows of large oblong square blotches of dark brown on the back, is
found in the mountains of eastern United States. The little pale
reddish-brown wood-frog (_R. sylvatica_) with arms and legs barred
above is common in damp woods and is "an almost silent frog." The
peculiar and infrequently seen frogs known as the "spade-foots"
(_Scaphiopus_) are subterranean in habit and usually live in dry
fields or even on arid plains and deserts. They pass through their
development and metamorphosis very rapidly, appearing immediately
after a rain and laying their eggs in temporary pools. At this time of
egg-laying they utter extraordinarily loud and strange cries. Some
frogs in other parts of the world live in trees, and the eggs of one
species are deposited on the leaves of trees, leaves which overhang
the water being selected so that the issuing young may drop into it.
The true tree-frogs or tree-toads (Hylidæ) constitute a family
especially well represented in tropical America. They have little disk-
or pad-like swellings on the tips of their toes to enable them to hold
firmly to the branches of the trees in which they live. Some, like the
swamp tree-frog and the cricket-frog, are not arboreal in habit,
remaining almost always on the ground. The common tree-frog of the
eastern States (_Hyla versicolor_) is green, gray, or brown above with
irregular dark blotches, and yellow below. It croaks or trills,
especially at evening and in damp weather. Pickering's tree-frog (_Hyla
pickeringii_) makes the "first note of spring" in the eastern States.
This tree-frog is the one most frequently heard in the autumn too, but
"its voice is less vivacious than in the spring and its lonely pipe in
dry woodlands is always associated with goldenrods and asters and
falling leaves." The tree-frogs of North America lay their eggs in the
water on some fixed object as an aquatic plant, in smaller packets than
those of the true frogs, and not in strings as do the toads.
The toads (Bufonidæ) differ from the true frogs in having no teeth and
in not having, as the frogs do, a cartilaginous process uniting the
shoulder-bones of the two sides of the body. The absence of this
uniting process makes the thoracic region capable of great expansion.
There are only a few species of toads in North America, but one of
these species, the common American toad (_Bufo lentiginosus_), is very
abundant and widespread. It appears also in two or three varieties,
the common toad of the southern States differing in several
particulars from that of the northern. The toad is a familiar
inhabitant of gardens, and does much good by feeding on noxious
insects. It is most active at twilight. Its eggs are laid in a single
line in the centre of a long slender gelatinous string or rope, which
is nearly always tangled and wound round some water-plant or stick
near the shore on the bottom of a pond. The eggs are jet black and
when freshly laid are nearly spherical. At the time of egg-laying the
toads croak or call, making a sort of whistling sound and at the same
time pronouncing deep in the throat "bu-rr-r-r-r." The toad does not
open its mouth when croaking, but expands a large sac or resonator in
its throat. The toad-tadpoles are blacker than those of frogs or
salamanders, and undergo their metamorphosis while of smaller size
than those of frogs. When they leave the water they travel for long
distances, hopping along so vigorously that in a few days they may be
as far as a mile from the pond where they were hatched. They conceal
themselves by day, but will appear after a warm shower; this sudden
appearance of many small toads sometimes gives rise to the false
notion that they have fallen with the rain.
=Cœcilians (Gymnophiona).=--The third order of batrachians, the
cœcilians, includes about twenty species of slender worm- or
snake-like limbless forms which are confined to the tropics. Some of
them are wholly blind and the others have only rudimentary eyes. In
them the skin is folded at regular intervals so that the body appears
to be rigid or segmented, and in some species there are small
concealed horny scales in the skin.