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metamorphoses, commencing their existence immediately after leaving the egg in
a larval condition, during which they breathe the air contained in water by means
of gills, while in the adult state they breathe atmospheric air by means of lungs.
Varying much in external form, these animals nearly always have the body
covered with a soft naked skin ; but in a few instances among existing forms scales
are embedded in the skin, and most of the extinct forms had a well-developed
armour of scales and bony scutes. In some forms a longitudinal fin is developed
down the middle of the back and tail, but this is always soft, and lacks the support-
ing spinous bones characterising that appendage in fishes. In passing through a
metamorphosis, Amphibians are more like the inferior groups of animals than the
higher Vertebrates ; and while in the earlier stages of their existence, during which
they breathe by gills, they may be regarded as very closely allied to Fishes, in the
adult state they come much nearer to Reptiles. The extinct Labyrinthodonts,
which are themselves not very widely removed from fishes, and have the basi-
occipital bone ossified, serve to connect other members of the class with the
Anomodont and Beaked Reptiles. And it may be mentioned here that while in
Mammals the skull has continued to be supported by the two condyles of the
Amphibians, in the Reptiles the basioccipital bone has developed an intermediate
condyle filling up the gap between the two exoccipital condyles, and thus forms
a single tripartite condyle like that of the tortoises. Frequently, as in the
crocodiles, the lateral elements have tended more or less completely to disappear,
thus leaving a condyle formed almost entirely by the basioccipital.

As already said, the skin of most existing Amphibians is soft and
naked ; it is invested with a colourless epidermis, which is periodically
shed entire, while the deeper layer is often coloured with blotches or streaks of
yellow, red, brown, or black. Other colours, however, such as green and blue, are
produced by pigment-cells, which generally make their appearance under special
conditions of warmth and moisture. As a rule, the colour of Amphibians varies to
a great extent with the nature of their surroundings, as is well exemplified in the
case of the frog, which changes its hue according to the nature of its habitat ; while
the tree-frogs harmonise with the foliage among which they dwell. It is, however,
very remarkable that in Costa Rica a certain toad simulates to an extraordinary
degree the coloration of the snakes both poisonous and harmless of the same
country ; while in North Sumatra Amphibians of various groups are spotted with
carrnine-red. In all Amphibians the skin is furnished with glands secreting a
more or less milk-like fluid; these glands being generally distributed all over
the body, although sometimes they are confined to the sides of the neck behind the
eyes. In many toads and land-salamanders some of the larger glands appear as
prominent warts, pierced with large pores. The viscid, milky fluid secreted by
these glands is exuded during excitement, and is endued with more or less
poisonous properties, being intended to serve as a means of defence. Although
some degree of irritation of the skin may be produced by handling some of
the species in which these poisonous properties are most developed, the stories
of toads or salamanders spitting venom are, it is almost needless to observe,
pure fabrications. When introduced into the circulation, batrachian venom acts,
however, as a powerful poison, influencing the heart and central nervous .system ;



GENERAL CHARACTERISTICS.



2 59



and the secretion of a South American species is employed by the Indians to poison
spears and arrows used in killing monkeys.




In the economy of Amphibians the naked skin and its glands play a most
important part, since none of them drink, in the proper sense of the word,



2 6o AMPHIBIANS.

but imbibe moisture through the pores of their integument. Moisture is, indeed,
essential to their existence, and if they be confined in a dry atmosphere they
soon perish. It is true that frogs may be seen basking in the sun's rays, and
apparently enjoying the warmth as much as lizards, but they only do this in the
neighbourhood <>!' water, to which they retire when necessary. Such members of
the class as inhabit dry localities, are mostly nocturnal, avoiding sunshine, and
wandering abroad when they can obtain moisture from dew,

The skeleton of the Amphibians presents many peculiarities, and
in some forms has numerous fish-like characters. For instance, in
certain of the forms with permanent gills the vertebra? are scarcely distinguishable
from those of fishes ; whereas in the true newts they have a rounded knob at the
front of the body and a cup at the hinder extremity, and are closely articulated
with one another. In the long-tailed groups the number of vertebrae is consider-
able ; but in the frogs and toads those of the back are reduced to seven or eight,



Skeleton.







SKELETON OP SALAMANDER.



the hinder-end of the backbone terminating in a long style, extending between the
greatly produced extremities of the haunch-bones, or ilia, which articulate with the
lateral processes uf the sacral vertebrae. The transverse processes of all the
vertebrae are well-developed, and in some cases very long ; and they take the pit
of ribs, which, at the most, are represented by some small rudiments. In cor
quence of this absence of ribs, Amphibians are unable to breathe in the ordin;
way by alternate expansion and contraction of the cavity of the chest; and
they, so to speak, swallow air, taking in a large gulp, and then closing the
mouth. In addition to the peculiarities connected with its condyles and (he
basioccipital region, the skull is distinguished by its flattened, broad, and more or
less semicircular form ; the sockets for the eyes being generally large and ill-define
In front of the condyles the under surface of the middle of the skull is overlain
a large parasphenoid bone, which is frequently dagger-shaped ; this bone beii
generally but slightly, if at all, developed in the higher Vertebrates, although vei
large in Fishes. The lower jaw, which articulates with the skull by the intervei
tion of a quadrate-bone, is composed of at least two pieces on each side, and may



SKELETON.



261




SKELETON OF FROG.



1



contain more elements. The palatines and vonier, and more rarely the parasphenoid,
may be armed with teeth, like the upper jaw ; but in the frogs and toads the lower
jaw is very generally toothless. In all cases the teeth are small, simple, and pointed ;
being adapted for holding, and not for masticating. The shoulder-girdle, which is
largely cartilaginous, is placed
very close to the head, and com-
prises the usual elements, Each
scapula, or shoulder-blade, has an
upper cartilaginous portion, ex-
tending inwards nearly to the
middle line of the back ; while in
the frogs each metacoracoid has
an inward cartilaginous expan-
sion, which may either meet or
overlap its fellow, and is of much
importance in classification. In
advance of the metacoracoids is
another pair of transverse bars
commonly known as the precora-
coids ; while in front of these is a

single median rod termed the omosternum ; the proper sternum, or breast-bone,
occupying a similar position behind the metacoracoids. In the fore-limb the radius
and ulna may be united, and the wrist cartilaginous ; the number of toes among
living forms never exceeding four, and being sometimes reduced to three. More
variation exists in the hind-foot, the number of toes in the long-tailed forms
ranging from two to four, whereas in the frogs and toads it is always five. Only
in a few frogs and newts are the toes furnished with claw-like nails; in the
greater number of forms these being naked, although often connected by webs, and
sometimes carrying adhesive discs on the lower surface.

In all Amphibians the brain is of a very low type, its component
portions lying in a line one behind the other, without overlapping.

il possess the three chief organs of sense, although in some instances the eyes
be very minute and covered with an opaque skin. In frogs and toads the

r e is large and very highly developed ; generally possessing two lids, of which the
lower one is larger and thinner 'than the upper, and more or less transparent.
Greater variation exists in the structure of the ear, which is simplest in the
tailed forms. The nose opens externally in a pair of nostrils situated near the
muzzle, and by another pair of apertures into the mouth ; the latter character
distinguishing Amphibians from the majority of Fishes. The tongue, which acts
only in the very slightest degree as an organ of taste, and is wanting in one group
of frogs, is generally well-developed and thick, filling the whole space between the
jaws, and being capable of a large amount of motion ; it differs essentially from
that of the higher Vertebrates in that it is affixed to the inner side of the front of
the lower jaw, with its tip pointing down the throat.

All Amphibians lay eggs, which are generally although not
invariably deposited in fresh water, and fertilised as they are



Soft Parts.



Development.



262



AMPHIBIANS.






extruded from the female. As a rule, these eggs, which much resemble those of
fish, are small, very numerous, and connected together by mucilage, forming either
a string or a jelly-like mass in which the dark yolks are very conspicuous. Some
of the tree-frogs, however, lay large eggs, within which the larvae undergo the
whole of such transformation as takes place; and in one genus, instead of the
usual gills, a temporary breathing-organ is developed on the tail. A land-frog in
the Solomon Islands also lays large eggs, like small marbles, which are deposited
in the crevices of rocks, and from which emerge fully-developed frogs. The eg^s,
with certain exceptions, are deposited in water, where they are hatched by
the heat of the sun ; and it appears that the dark colour of the yolk is for
the purpose of absorbing as much solar heat as possible. Such eggs as are
laid during the late spring and summer are less darkly coloured, and have.
thinner coats, than those deposited in the early part of the spring; and while tin-








DEVELOPMENT OK THE FROG.



1, Eggs when first laid ; 2, Eggs at a later stage ; 3, Egg containing embryo ; 4, Newly-hatched larva- ;
5, 6, Larvae with external gills ; 7-12, Later stages in the development of larvae.

former are placed on the ground at the bottom of the water, the latter float on the
surface; the reason of this difference being that in the early part of the year
the lower strata of water are too cold to admit of the development of the ova.
In ordinary cases, when the larva has reached a certain stage, it bursts tin-
investing membranes of the egg, and comes into the world adapted for an
aquatic life, and always possessing a long compressed tail composed of zigzag-
shaped masses of muscles, similar to those of fishes. The first process is Ilie
sprouting forth of branching external gills from the sides of the neck, which in
the larvae of the frogs and toads are subsequently replaced by internal gills, but in
the long-tailed forms persist for a longer period. After the disappearance of the
external gills, the water is expelled From the gill-chamber by one or two tubes,
generally discharging by a single orifice, which may be situated either on the
lower surface of the body, or on the left side. As soon as the external gills have
made their appearance, development is concentrated on the tail and the absorption



DISTRIBUTION. 263

of the remainder of the yolk. The vertical fin-like expansions of the tail rapidly
increase, and the body becomes relatively smaller and more slender; while the
limbs begin to make their appearance as buds, although the date of development
of the front and hind-pair varies in different groups. In the newts, the front
pair of limbs are the first to appear, in the frogs the reverse is the case. In
the latter the hind-limbs appear some considerable time before the front pair,
the fish-like tail persisting till the sprouting of these, when the change from
a herbivorous fish-like animal to one carnivorous and reptiliform begins. The
jaws are at first invested with horny teeth, and subsequently with horny
sheaths, which eventually disappear ; while the tail gradually diminishes in size,
and finally is lost. It may be observed that no vertebrae are developed in the
frog's tail ; and that the long spine in which the backbone of the adult terminates
is an outgrowth from the hindmost vertebra. Not less remarkable is the
shortening of the intestinal canal, as the creature changes its herbivorous for
carnivorous habits. To trace in detail the development of the soft parts would
greatly exceed our limits of space. We may mention, however, that in one group
of Tailed Amphibians the external gills of some individuals may be retained
permanently, while in others of the same species they are cast at an early period.
Then, again, the number of these gills is by no means constant, for in the Cingalese
ca3cilian and the salamander there are three pairs of these organs, in the tadpoles
of some frogs there are two, and in others, as well as in one genus of csecilians,
there are only a single pair.

Geologically the Amphibians are a very ancient group, their
oldest representatives occurring in the Carboniferous and Permian
iks of Europe and North America. All these ancient representatives of the
lass belong, however, to the group of Labyrinthodonts, which survived till the
period of the Trias, and are structurally very different from the modern forms,
approximating in certain respects to fishes. Indeed, since no Amphibians have
hitherto been discovered between the Trias and the Wealden, or lower Cretaceous,
rocks of Belgium, we are quite unable to assert that the modern representatives of
the class are the direct descendants of the Labyrinthodonts. Commencing in the
Belgian Wealden, the newts and salamanders occur throughout the greater part of
the Tertiary rocks ; but the frogs and toads are first known in North America
from Eocene beds, while in Europe they are not met with before the Oligocene.

At the present time Amphibians are distributed over all parts of the world
except the polar regions ; although they are more dependent upon the presence
of water and warmth than any of the preceding classes of Vertebrates. They are,
accordingly, most abundant in the tropical and subtropical regions ; and as none
of them are marine in their habits, even a narrow arm of the sea is generally
sufficient to limit their habitat. When they occur on islands, it is probable either
that their eggs have been carried by birds, or that there has been a comparatively
recent separation from the mainland. In absolutely desert districts Amphibians
are unknow T n ; while in countries where there is a long dry season, followed
by a period of rains, they are in the habit of becoming torpid during the
former ; the length of the sleep in one Javan species being upwards of five months.
In cold climates all the members of the class become torpid during the winter.



264



AMPHIBIANS.



As regards their general distribution, Amphibians closely resemble fresh-water
tish, and differ widely from li/ards. Indeed, from an Amphibian point of view,
the globe may be divided into two great regions, namely, a northern one
characterised by the abundance of newts and salamanders, and the absence of
caecilians ; and a southern one distinguished by the want of the former and the
presence of the latter group.

In their mode of life, it is probable that very few Amphibians

are diurnal ; most of the terrestrial forms making their appearance

abroad with the first shades of evening, and retiring to their hiding-places at

dawn. In wet or cloudy weather frogs and toads especially in South America

frequently appear in great numbers during the day ; and both these groups are




MOOR-KROGS (nat. size).

in the habit of making night hideous with their croakings. Although in all cases
the adults are carnivorous, the larvae subsist more or less exclusively on vegetable
substances; some confining themselves to that kind of diet, while others also
consume animalcules and other minute creatures.

Characteristics of The f rogs and toads are distinguished from their allies by the
Frogs and Toads. presence of four limbs and the absence of a tail in the adult stair ;
the latter feature giving origin to the name Ecaudata, by which the order to
which they belong is scientifically designated. They all have short and frequently
thick bodies, in which the backbone comprises, at most, only eight vertebra 1 in
advance of the sacrum; those behind the latter being fused into a long rod-like
bone, as shown in the figure of the skeleton on p. 261. In the fore-limb, as sh<>\\ n
in the same lio-inv, the bones of the fore-arm (radius and ulna) are completely fused
together; and the same is the case with regard to the tibia and fibula in the hind-



TYPICAL FROGS. 265

leg. Moreover, the hind-limb obtains a kind of additional segment, owing to the
elongation of the calcaneum and astragalus in the ankle-joint, which form a pair
of long bones lying parallel to one another. As a rule, frogs and toads undergo a
lengthened larval period ; the " tadpoles," as shown in the figure on p. 262, having
a globular head and body, a fish-like tail, external or internal gills, and no limbs in
the first stages of their existence. The hind-limbs are the first to appear, and
after the front pair are developed the tail is gradually absorbed, upon which the
young for the first time leave the water. Represented by about a thousand species,
frogs and toads have a worldwide distribution, although more abundant in tropical
and subtropical than in temperate regions, and being especially numerous in India
and South America ; and it is not a little remarkable that some of the largest
forms are inhabitants of islands. From the nocturnal habits of the adults it is
frequently difficult to find out whether in any locality these reptiles are abundant
or the reverse ; but in the spring this may generally be ascertained by observing
the tadpoles in the rivers and points, since all of these show specific differences, to
the full as well marked as those in the adult.



THE TYPICAL FROGS.
Family RANID^E.

The typical frogs, together with four other families, constitute a suborder
( Firmisternia), characterised by the presence of a tongue, and by the firm union
of the two metacoracoid bones of the chest by means of a single cartilage uniting
their free edges. From the other members of the group, the typical frogs are
distinguished as a family by the presence of teeth in the upper jaw, and by the
transverse processes of the sacral vertebra being either cylindrical, or but very
slightly dilated at their extremities. These characters are sufficient to distinguish
the typical frogs from the other families of the suborder ; but it may he added that
the vertebrae are cupped in front and hollowed behind ; while there are no ribs ;
and the terminal style of the backbone is articulated to the sacrum by two
condyles. The terminal joints of the toes may be either simple or pointed,
T-shaped, Y-shaped, or even claw-like ; the species in which these joints are thus
expanded having the soft parts similarly expanded and flattened. For a long time
it was considered that the shape of the tips of the toes was connected with the
mode of life of their owners ; and although this is so to a great extent, it is now
ascertained that several of the species in which the toes are somewhat expanded
are as aquatic as those in which they are pointed, and species presenting both
modifications are included within one and the same genus. The typical frogs are
divided into twenty genera, only two of which are noticed in this work.

Under the general title of water-frogs may be conveniently
Water-Frog's

included all the members (some hundred and forty in number), of the

genus Rana, to which belongs the common English frog. The distinctive characters
of these frogs are to be found in the horizontal pupil of the eye; the more or
less deeply notched and free tongue ; the presence of teeth on the vomerine bones
of the palate ; the absence of webs in the toes of the fore-feet, and their presence



266 FROGS AND TOADS.

in those of the hind-limb ; and the separation of the outer metatarsal bones of the
hind-foot by a web, the extremities of the fingers being simple or expanded.

With the exception of the southern part of South America (where the whole
family is unrepresented), Australia, and New Zealand, these frogs have a worldwide
distribution. Although the greater majority of the species are probably aquatic
during the breeding-season, at other times great diversity of habit is displayed by
the different representatives of the genus, some being aquatic, others terrestrial,
and others, again, burrowing, or even more or less arboreal. The existence of
burrowing habits is indicated by the great development of a tubercle on the inner
side of the metatarsus, which in one Indian species (Rana breviceps) has a sharp
edge, and is used in a shovel-like manner to excavate the burrow. Such burrow-
ing species are further characterised by the shortness of the hind-limbs, and thus
assume a more or less toad-like appearance. Large discs at the ends of the toes
usually, on the other hand, are indicative of arboreal habits ; although, as already
said, smaller discs are met with in certain purely aquatic species.

Selecting some of the European representatives of the genus for
special mention, we may first notice the edible frog (R. esculenta),
characterised by the pointed tips of the. toes, the smooth under surface of the body,
the presence of a broad glandular fold along the sides, and the marbling of the
thighs. Exceedingly variable in coloration, this frog generally has the upper-parts
olive or bronzy brown, more or less spotted or marbled with dark brown or black ;
there are generally three light stripes along the back, while the sides of the IK-MI 1
and ground-colour of the flanks are sometimes green; the marbling on the thighs
occupying their hinder surfaces, and being black in colour. The males are specially
characterised by the presence of a globular sac, connected with the production of
the croaking, on each side of the head, opening by a slit behind the angle of the
mouth. Inhabiting Europe, Asia as far west as Japan, and North- Western Africa.
the edible frog is common in England, the dark race occurring in the fens of
Cambridgeshire, and the green variety in Norfolk. The use of the flesh as food
probably led to the introduction of this species into Cambridgeshire by the monks ;
while the Norfolk colony was imported between 1837 and 1842. From this species
the common English frog (R. temporia) is readily distinguished by the incomplete
webbing of the hind-feet, and the presence of a dark temporal spot extending from
the eye to the shoulder, as well as by the absence of external vocal sacs in the
males. Moreover, if the skulls of these two species be compared, it will be found
that while in the edible frog the teeth on the vomers do not extend behind tl
line of the apertures of the posterior nostrils, they do so to a small extent in tl
present species. In colour the upper-parts of the common frog are greyish
yellowish brown, more or less spotted with dark brown or black ; the tempoi
spot being always dark, and a light line running from below the eye to it
extremity ; while the sides of the body are profusely spotted, the limbs trai
versely barred, and a larger or smaller number of spots are present on the undei
parts. This species is spread over Europe and Northern and Temperate ASIJ
Closely allied is the moor-frog (R. arvalis), of Eastern Europe and Wr
Asia, represented in the illustration on p. 264, which may be distinguished by tl
tubercle on the inner metatarsal being compressed instead of blunt, and by tl



TYPICAL FROGS.



267



pointed, in place of obtuse, muzzle. The coloration is very similar to that of the
common species, but there is sometimes (as in the right-hand figure of the illustra-
tion), a light stripe bordered by two black ones down the middle of the back, while
the under-parts are uniform. A third European species is the agile frog (R. agilis),
which belongs to a group distinguished by the greater length of the hind-



Online LibraryRichard LydekkerThe new natural history (Volume 5) → online text (page 28 of 62)