AMPHIBIANS--A CONNECTING LINK
We have now come to a class of vertebrates that in their manner of
life, and presumably in their history, connect the dwellers in the
waters with those on the lands of the globe. Dr. Gamble cites examples
from various groups of animals to show that adaptation to a terrestrial
existence is an advance on that requisite for aquatic life, and that
the critical point in the evolution of the vertebrate phylum was passed
when its members migrated from water to land. "When we come to land
animals," he says, "the problem of weight has to be considered before
that of locomotion. The lateral undulations of the body, even when
aided by unjointed paddles, or fins, are not sufficient to insure rapid
movement on land. Hence a system of levers has to be evolved, partly to
support the body, and partly to propel it. The use of joints becomes
a necessity, and we find that all active terrestrial animals, except
snakes, have jointed limbs. The critical point in the history of this
phylum is passed when its members migrated from water to the land. The
step was taken by the ancestors of the Amphibia (that is, the frogs,
toads, and salamanders). In them the breast fins of the fish have
become the jointed forelegs, the pelvic fins have become the hind legs."
How this great change from the fish fin to the five-fingered hand
occurred is, at present, just as obscure as the mode of conversion
of the arms of reptiles into the wings of birds. The answer can only
be supplied by further discoveries in the geological history of the
order, and though this history can be traced back to the time of the
Coal Measures, we find the earliest Amphibia as sharply marked off
from the fishes by their feet as they are to-day. These forefathers
(subclass Stegocephalia) are the earliest known four-footed animals,
and their fossil skeletons are found from the Carboniferous up to the
Trias, after which the race disappears. They had the general form of
newts, and many were only a few inches in length. That some of these,
at least, were terrestrial in habit is shown by the fact that they are
often found in stumps and hollow logs of sigillarias and other fossil
trees of the coal beds, especially in Nova Scotia. But there were also
species several feet in length, with formidable teeth, which were no
doubt carnivorous and predatory, so that it was well for the little
ones to seek places of safety. These stegocephalians were unmistakably
amphibians, with two condyles supporting the skull, but their skeleton
contains many features that suggest reptilian anatomy, and it is agreed
that the reptiles sprang from this stock. The peculiar feature of this
group is that their flattish heads were covered by a broad shield of
bony plates (ossified skin); and similar armor protected their bellies,
and in a few cases the back also.
Geological formations furnish no ancestral connection between the
Stegocephalia and modern salamanders; but the limbless, wormlike,
burrowing and blind cæcilians of the tropics exhibit certain
stegocephalian characteristics, especially a scaly skin, which put them
into a division (Apoda) by themselves. The remainder of the class,
that is, Amphibia (also called Batrachia) in general, have a soft,
moist, naked skin, and are naturally divisible into two orders:
1. _Urodela_--Tailed amphibians: newts and salamanders.
2. _Anura_--Tailless amphibians: frogs and toads.
Modern amphibians in general are animals fitted for life both on land
and in water. All are born from eggs hatched in water, and the young,
at first in a larval form unlike the adult condition, have external
gills adapted to breathing in that element; but in most cases they
lose their gills, and as adults acquire lungs for breathing air. This
metamorphosis of the young, comparable to that of the nymph-producing
insects, is the especial characteristic of the class. The skeleton is
of the vertebrate model, but in the Urodela is largely cartilaginous.
The skin is smooth, soft, moist, and covered only with a filmy coat
of horny texture that is molted from time to time as the animal needs
room to grow. The skin abounds in sense organs about the head and
along the sides of the body--an inheritance from the lateral line of
fishes--which are most active in the larvæ, and disappear altogether
with age in most frogs and toads, although they revive in salamanders
in the breeding season.
The skin also contains many mucus glands and other larger glands,
especially on the back. These emit under provocation a poisonous liquid
that is fatal to small animals, and very irritating to the eyes, nose,
and throat of larger ones. Most, if not all, Amphibia, says Dr. Gadow,
are more or less poisonous, and it is significant that many of the most
poisonous exhibit a very conspicuous yellow or orange upon a dark
ground, which is so widespread a sign of poison. There is no venom in
their bite--in fact, their teeth are too small, although numerous, to
let anyone fear their biting. The skin is heavily laden with pigment,
and this is displayed in many amphibians in striking patterns of bright
coloring. Certain groups possess in a high degree the power of altering
their colors to conform to their surroundings.
An interesting feature of the amphibians is that power of repairing
mutilations of the body and replacing lost parts which is so well
known in worms, hydroids, and other lowly creatures, and is termed
"regeneration." This ability is most active in young specimens.
Tadpoles frequently have their tails bitten off, whereupon new ones
grow quickly. Salamanders fight bitterly, tearing off each other's
gills and limbs, and turtles and fishes frequently bite off their
tails. New tails are generated speedily, and usually in good and
effective form, although they contain no regenerated caudal vertebræ,
but only a rod of cartilage. The ability to rebuild lost parts is much
less among the frogs.
Another notable fact is that here for the first time we meet with a
voice organ, and a real voice expressing emotions, although in an
extremely limited way. This is most noticeable in the tree frogs, which
are the most advanced of the Amphibia in organization.
NEWTS AND SALAMANDERS
The Urodela are represented throughout the whole northern hemisphere
except in desert regions, as far in North America as southern Canada,
and also southward to Panama; and in the Old World, northward to
the line of very cold winters and southward to the Mediterranean and
Indo-China. In the main, however, our genera are different from those
of Europe and Asia.
The largest and best known of American urodeles is a member of the
family Proteidæ and genus Necturus, and is widely known as "water
dog" or "mud puppy," because of the doglike shape of its head. It is
a brown, robust creature, sometimes two feet long, with bushy gills,
retained throughout its life, springing from open gill clefts in
three bright red tufts on each side of the head. It inhabits cold,
rapid streams, hiding under stones by day, and moving about at night
in search of crawfish, worms, insect larvæ, frogs, etc., and dodging
hungry snapping turtles. But little smaller, and even more ugly in
appearance, is the "hellbender," representing the family Amphiumidæ.
These blackish creatures are to be found in mountainous regions, and
hide during the day under loose rocks. By the time they are about
three years old their gills have been absorbed, and their lungs are in
service, so that they are compelled to rise to the surface occasionally
for drafts of air. They hunt at night for food, preferring crawfish
and, fishermen say, fish eggs. The breeding habits of this animal
have only lately become known, and Mr. B. G. Smith, who has made a
special investigation of them, says that the breeding season begins (in
Pennsylvania) in August, when hellbenders of both sexes come out more
freely from their rock shelters and roam about, frequently in small
companies. The small number of eggs produced are hidden in a pocket
under a loose stone; and the young, which are more like tadpoles than
the form of their parents, breathe by gills which do not completely
disappear until the animals have reached nearly their maturity.
Otherwise our salamanders are small species found in brooks, ponds, and
wet woods, and often getting into cellars and wells. Uninformed persons
think them to be lizards, and foolishly fear them, but except for the
irritation of the hands that may follow rough handling they are utterly
harmless to man or his property, and serve him by devouring great
quantities of insects and worms.
A common species in damp, neglected woodlands is the little red-backed
fellow that is so light and leaping in its movements when disturbed,
even throwing off its tail in its panic of fear. It is more terrestrial
than most, laying its few eggs in rotting wood instead of going into
the water for that purpose; and the young carry gills but a few days.
This red-backed Plethodon must not be confused with the small newts,
bright vermilion with a row of glowing spots along the sides, that are
found in woods in summer. They are young specimens of _Diemyctylus
viridescens_, which is common all over the eastern part of the United
States and southern Ontario. The parents are green, and wholly aquatic
in habits. The larvæ have gills and swim about until early autumn,
by which time their gills have been gradually absorbed, and they go
ashore, where their coats change in color from a mottled green to
scarlet. This red condition and their residence on land continue until
the autumn of the third, or the spring of the fourth year of their
lives, when they become sexually mature, resume a greenish dress, go
back to the water, and pass the rest of their lives there.
Mention can be made of only one more species--the black, yellow-spotted
"tiger triton," which is the most widely spread and often seen of
our terrestrial salamanders. It is especially noteworthy because of
the extraordinary condition of suspended development exhibited by its
larva, the famous edible axolotl of Mexican lakes, which, while still
retaining larval gills and aquatic habits, grows nearly or quite to
the size of its parents--three to four inches--and becomes capable of
breeding. Similar cases are known in certain lakes in southern Europe;
and it appears that this arrested development, together with natural
growth of body, occurs occasionally in many other amphibians. The
condition is termed "neotony," but the biological explanation of it is
not clear.