BATRACHIA
The theory of evolution teaches that animal life began in a very simple
form in the sea, and that afterward the higher sea animals lost their
gills and developed lungs and legs and came out to live upon the land;
truly a marvellous procedure, and incredible to many, although the
process is repeated every spring in countless instances in pond and
brook.
In popular language, every cold-blooded vertebrate breathing with lungs
is called a reptile. The name reptile is properly applied only to
lizards, snakes, turtles, and alligators. The common mistake of speaking
of frogs and salamanders as reptiles arises from considering them only
in their adult condition. Reptiles hatch from the egg as tiny reptiles
resembling the adult forms; frogs and salamanders, as every one knows,
leave the egg in the form of tadpoles (Fig. 248). The fact that frogs
and salamanders begin active life as fishes, breathing by gills, serves
to distinguish them from other cold-blooded animals, and causes
naturalists to place them in a separate class, called batrachia (twice
breather) or amphibia (double life).
TADPOLES
SUGGESTIONS.—Tadpoles may be studied by placing a number of frog’s
eggs in a jar of water, care being taken not to place a large number
of eggs in a small amount of water. When they hatch, water plants
(_e.g._ green algæ) should be added for food. The behaviour of frogs
may be best studied in a tub of water. A toad in captivity should be
given a cool, moist place, and fed well. A piece of meat placed near a
toad may attract flies, and the toad may be observed while catching
them, but the motion is so swift as to be almost imperceptible. Live
flies may be put into a glass jar with a toad. Toads do not move about
until twilight, except in cloudy, wet weather. They return to ponds
and brooks in spring at the time for laying eggs. This time for both
frogs and toads is shown by trilling. All frogs, except tree frogs,
remain in or near the water all the year.
[Illustration:
FIG. 248.—METAMORPHOSES OF THE FROG, numbered in order.
]
Do =eggs hatch= and tadpoles grow more rapidly in a jar of water kept in
a warm place or in a cold place? In pond water or in drinking water? Can
the tadpoles be seen to move in the eggs before hatching? When do the
external gills show? (Fig. 248.)
[Illustration:
FIG. 249.—TADPOLE, from below, showing intestine and internal gills.
(Enlarged.)
]
What =parts= may be described in a tadpole? What is the shape of the
tail? _Compare the tadpole with the fish_ as to (1) general shape, (2)
covering, (3) fins, (4) tail, (5) gills.
Do the external =gills= disappear before or after any rudiments of limbs
appear? (6, 7, Fig. 248.) Can you locate the gills after they become
internal? (Fig. 249.)
In what state of growth are the _legs_ when the tadpole first goes to
the surface to breathe? Which legs appear first? Of what advantage is
this? What becomes of the tail? Is the tail entirely gone before the
frog first leaves the water? Are tadpoles habitually in motion or at
rest?
Is the =intestine= visible through the skin? (Fig. 249.) Is it straight
or coiled? Remembering why some fish have larger intestines than others,
and that a cow has a long intestine and a cat a short one, state why a
tadpole has a relatively longer intestine than a frog.
=Compare= the mouth, jaws, eyes, skin, body, and habits of _tadpole and
frog_.
FROGS
Prove that frogs and toads are _beneficial to man_. Did you ever know of
a frog or a toad destroying anything useful, or harming any one, or
causing warts? How many pupils in class ever had warts? Had they handled
frogs before the warts came? Frogs are interesting, gentle, timid
animals. Why are they repulsive to some people?
=Environment=.—_Where are frogs found_ in greatest numbers? What occurs
when danger threatens them? What _enemies_ do they have? What colour, or
tint, is most prominent on a frog? Does the colour “mimic” or _imitate_
its surroundings? What is the colour of the under side of the body?
(Fig. 250.) Why is there greater safety in that colour? What enemies
would see water frogs from below? Do tree frogs mimic the bark? The
leaves?
Can a _frog stay under water_ for an indefinite time? Why, or why not?
What part of a frog is above the surface when it floats or swims in a
tub of water? Why? Do frogs croak in the water or on the bank? Why do
they croak after a rain? Do toads croak?
Are the _eggs_ laid in still or in flowing water? In a clear place or
among sticks and stems? Singly, or in strings or in masses? (Fig. 248.)
Describe an egg. Why do frogs dig into the mud in autumn in cold
climates? Why do they not dig in mud at the bottom of a pond?
[Illustration:
FIG. 250.—PAINTED FROG (_Chorophilus ornatus_), of Mexico.
]
Describe the =position= of the frog when still (Fig. 250). Of what
advantage in this position? Does the frog use its fore legs in swimming
or jumping? Its hind legs? How is the frog fitted for jumping? Compare
it in this respect with a jumping insect; a jumping mammal. How is it
fitted for swimming? Is the general build of its body better fitted for
swimming or for jumping? How far can a frog jump?
=External Features.=—The frog may be said to have two _regions in its
body_, the head and the trunk. A neck hardly exists, as there is only
one vertebra in front of the shoulders (Fig. 252), while mammals have
seven neck (cervical) vertebræ. There are no tail (caudal) vertebræ,
even in the tadpole state of frogs and toads.
The _head_ appears triangular in shape when viewed from what direction?
The head of a frog is more pointed than the head of a toad. Is the skull
a closed case of broad bones or an open structure of narrow bones? (Fig.
252.)
[Illustration:
FIG. 251.—HEAD OF FROG.
]
Describe the _mouth._ Observe the extent of the mouth opening (Fig.
251). Are _teeth_ present in the upper jaw? The lower jaw? Are the teeth
sharp or dull? Does the frog chew its food? Is the _tongue_ slender or
thick? (Fig. 251.) Is it attached to the front or the back of the mouth?
In what direction does the free end extend when the tongue lies flat? Is
the end pointed or lobed? How far out will the tongue stretch? For what
is it used? Why is it better for the teeth to be in the upper jaw rather
than in the lower jaw? That the teeth are of little service is shown by
the fact that the toad with similar habits of eating has no teeth. Will
a toad catch and swallow a bullet or a pebble rolled before it? The toad
is accustomed to living food, hence it prefers a moving insect to a
still one.
=The Senses.=—Compare the _eyes_ with the eyes of a fish in respect to
position and parts. Are the eyes protruding or deep-set? Touch the eye
of a live frog. Can it be retracted? What is the shape of the pupil? The
colour of the iris? Is the eye bright or dull? What probably gave rise
to the superstition that a toad had a jewel in its head? Is there a
third eyelid? Are the upper and lower eyelids of the same thickness?
With which lid does it wink? Close its eye?
Observe the large oval _ear_ drum or tympanum. What is its direction
from the eye? (Fig. 251.) The mouth? Is there a projecting ear? Does the
frog hear well? What reason for your answer? As in the human ear, a tube
(the Eustachian tube) leads from the mouth to the inner side of the
tympanum.
How many _nostrils_? (Fig. 251.) Are they near together or separated?
Large or small? A bristle passed into the nostril comes into the mouth
not far back in the roof. Why must it differ from a fish in this?
[Illustration:
FIG. 252.—SKELETON OF FROG.
]
How do the _fore and hind legs_ differ? How many toes on the fore foot
or hand? On the hind foot? On which foot is one of the toes rudimentary?
Why is the fore limb of no assistance in propelling the body in jumping?
Do the toes turn in or out? (Fig. 250.) How does the frog give direction
to the jump? What would be the disadvantage of always jumping straight
forward when fleeing? Which legs are more useful in alighting?
=Divisions of the Limbs=.—Distinguish the upper arm, forearm, and hand
in the fore limb (Figs. 252 and 253). _Compare with skeleton of man_
(Fig. 399). Do the arms of a man and a frog both have one bone in the
_upper arm_ and two in the _forearm_? Both have several closely joined
bones in the _wrist_ and five separate bones in the _palm_. Do any of
the frog’s fingers have three joints? _Compare also the leg of man_ and
the hind leg of the frog (Figs. 253 and 399). Does the _thigh_ have one
bone in each? The shank of man has two bones, shin and splint bone. Do
you see a groove near the end in the shank bone of a frog (Fig. 252),
indicating that it was formed by the union of a shin and a splint bone?
The first two of the five bones of the ankle are elongated, giving the
hind leg the appearance of having an extra joint (Fig. 253). The foot
consists of six digits, one of which, like the thumb on the fore limb,
is rudimentary. The five developed toes give the five digits of the
typical vertebrate foot. Besides the five bones corresponding to the
instep, the toes have two, three, or four bones each. How is the hind
foot specialized for swimming? Which joint of the leg contains most
muscle? (Fig. 254.) Find other bones of the frog analogous in position
and similar in form to bones in the human skeleton.
[Illustration:
FIG. 253.—SKELETON OF FROG.
]
[Illustration:
FIG. 254.—LEG MUSCLES OF FROG.
]
Is the =skin= of a frog tight or loose? Does it have any appendages
corresponding to scales, feathers, or hair of other vertebrates? Is the
skin rough or smooth? The toad is furnished with glands in the skin
which are sometimes swollen; they form a bitter secretion, and may be,
to some extent, a protection. Yet birds and snakes do not hesitate to
swallow toads whole. Show how both upper and under surfaces of frog
illustrate protective colouration.
[Illustration:
FIG. 255.—DIGESTIVE CANAL OF FROG.
_Mh_, mouth; _Z_, tongue pulled outward; _S_, opening to larynx; _Oe_,
gullet; _M_, stomach; _D_, intestine; _P_, pancreas; _L_, liver;
_G_, gall bladder; _R_, rectum; _Hb_, bladder; _Cl_, cloaca; _A_,
vent.
]
All batrachians have large and _numerous blood vessels in the skin_ by
which gases are exchanged with the air, the skin being almost equal to
_a third lung_. That the skin may function in this way, it must not
become dry. Using this fact, account for certain habits of toads as well
as frogs.
If a frog is kept in the dark or on a dark surface, _its skin will
become darker_ than if kept in the light or on a white dish. Try this
experiment, comparing two frogs. This power of changing colour is
believed to be due to the diminution in size of certain pigment cells by
contraction, and enlargement from relaxation. This power is possessed to
a certain degree not only by batrachians but also by many fishes and
reptiles. The chameleon, or green lizard, surpasses all other animals in
this respect (Fig. 280). What advantage from this power?
=Digestive System.=—The large mouth cavity is connected by a short
throat with the gullet, or œsophagus (Fig. 255). A slit called the
glottis opens from the throat into the lungs (Fig. 255). Is the gullet
long or short? Broad or narrow? Is the stomach short or elongated? Is
the division distinct between stomach and gullet, and stomach and
intestine? Is the liver large or small? Is it simple or lobed? The
pancreas lies between the stomach and the first bend of the intestines
(Fig. 255). What is its shape? A bile duct connects the liver with the
small intestine (_Dc_, Fig. 255). It passes through the pancreas, from
which it receives several pancreatic ducts. After many turns, the small
intestine joins the large intestine. The last part of the large
intestine is called the rectum (Latin, straight). The last part of the
rectum is called the cloaca (Latin, a drain), and into it the ducts from
the kidneys and the reproductive glands also open. The kidneys are
large, elongated, and flat. They lie under the dorsal wall. The urinary
bladder is also large. Does the salamander have a similar digestive
system? (Fig. 256) Why are the liver and the lungs (Fig. 256) longer in
a salamander than in a frog?
[Illustration:
FIG. 256.—ANATOMY OF SALAMANDER.
_1a_, heart; _2_, lungs; _3a_, stomach; _3b_, intestine; _3c_, large
intestine; _4_, liver; _8_, egg masses; _10_, bladder; _11_, vent.
]
=Respiration.=—How many _lungs_? Are they simple or lobed? (Fig. 256.) A
lung cut open is seen to be baglike, with numerous ridges on its inner
surface. This increases the surface with which the air may come in
contact. In the walls of the lungs are numerous capillaries. Does the
frog _breathe with mouth open or closed_? Does the frog have any ribs
for expanding the chest? What part of the head expands and contracts? Is
this motion repeated at a slow or a rapid rate? Regularly or
irregularly? There are valves in the nostrils for opening and closing
them. Is there any indication of opening and closing as the throat
expands and contracts? The mouth and throat (pharynx) are filled with
air each time the throat swells, and the exchange of gases (which
gases?) takes place continually through their walls and the walls of the
lungs. At intervals the air is forced through the glottis into the
lungs. After a short time it is expelled from the lungs by the muscular
abdominal walls, which press upon the abdominal organs, and so upon the
lungs. Immediately the air is forced back into the lungs, so that they
are kept filled. In some species the lungs regularly expand at every
second contraction of the throat. This is shown by a slight outward
motion at the sides. Does the motion of the throat cease when the frog
is under water? Why would the frog be unable to breathe (except through
the skin) if its mouth were propped open? Why does the fact that the
breathing is so slow as almost to cease during hibernation, aid the frog
in going through the winter without starving? (Chap. I.) Why must frogs
and toads keep their skins moist? Which looks more like a clod? Why?
=The Heart and Circulation.=—What is the shape of the heart? (Fig.
257.) Observe the two auricles in front and the conical ventricle
behind them. The great arterial trunk from the ventricle passes
forward beyond the auricles; it divides into two branches which turn
to the right and the left (Fig. 257). Each branch immediately
subdivides into three arteries (Fig. 257), one going to the head, one
to the lungs and skin, and a third, the largest, passes backward in
the trunk, where it is united again to its fellow.
[Illustration:
FIG. 257.—PLAN OF FROG’S CIRCULATION.
Venous system is black; the arterial, white. _AU_, auricles; _V_,
ventricle; _L_, lung; _LIV_, liver. Aorta has one branch to right,
another to left, which reunite below. Right branch only persists in
birds, left branch in beasts and man.
]
Both the pulmonary veins, returning to the heart with pure blood from
the lungs, empty into the left auricle. Veins with the impure blood
from the body empty into the right auricle. Both the auricles empty
into the ventricles, but the pure and the impure blood are prevented
from thoroughly mixing by ridges on the inside of the ventricle. Only
in an animal with a four-chambered heart does pure blood from the
lungs pass unmixed and pure to all parts of the body, and only such
animals are warm-blooded. The purer (_i.e._ the more oxygenated) the
blood, the greater the oxidation and warmth.
[Illustration:
FIG. 258.—FROG’S BLOOD (magnified 2500 areas). Red cells oval,
nucleated, and larger than human blood cells. Nuclei of two white
cells visible near centre. (Peabody.)
]
The red corpuscles in a frog’s _blood_ are oval and larger than those
of man. Are all of them nucleated? (Fig. 258.) The flow of _blood_ in
the web of a frog’s foot is a striking and interesting sight. It may
be easily shown by wrapping a small frog in a wet cloth and laying it
with one foot extended upon a glass slip on the stage of a microscope.
[Illustration:
FIG. 259.—BRAIN OF FROG.
]
[Illustration:
FIG. 260.—NERVOUS SYSTEM OF FROG.
]
The =brain= of the frog (Fig. 259) is much like that of a fish (Fig.
224). The _olfactory_, _cerebral_, and _optic lobes_, _cerebellum_ and
_medulla_ are in the same relative position, although their relative
sizes are not the same. Compared with the other parts, are the olfactory
lobes more or less developed than in a fish? The cerebral hemispheres?
The optic lobes? The cerebellum? There is a cavity in the brain. It is
readily exposed on the under surface of the medulla by cutting the
membrane, which is there its only covering (Fig. 259).
[Illustration:
FIG. 261.—Position of legs in tailless (_A_) and tailed (_B_)
amphibian.
]
=Frogs and toads are beneficial= (why?) and do not the slightest injury
to any interest of man. If =toads= are encouraged to take up their abode
in a garden, they will aid in ridding it of insects. A house may be made
in a shady corner with four bricks, or better still, a hole a foot deep
may be dug to furnish them protection from the heat of the day. A toad’s
muzzle is not so tapering as a frog’s (why?), its feet are not so fully
webbed (why?), and its skin is not so smooth (why?). In case of doubt,
open the mouth and rub the finger along the upper jaw; a frog has sharp
teeth, a toad none at all. The tadpoles of frogs, toads, and salamanders
are much alike. In toad’s spawn the eggs lie in strings inclosed in
jelly; frogs spawn is in masses (Fig. 248).
Any batrachian may easily be passed around the class after placing it
in a tumbler with gauze or net tied over top. It should be kept in a
box with two inches of moist earth on the bottom. If no live insects
are obtainable for feeding a toad, bits of moist meat may be dangled
from the end of a string. If tadpoles are placed in a pool or a tub in
a garden, the toads hatched will soon make destructive garden insects
become a rarity.
Does a frog or a =salamander= have the more primitive form of body? Why
do you think so? Salamanders are sometimes called mud puppies. The
absurd belief that salamanders are poisonous is to be classed with the
belief that toads cause warts. The belief among the ancients that
salamanders ate fire arose perhaps from seeing them coming away from
fires that had been built over their holes on river banks by travellers.
Their moist skin protected them until the fire became very hot.
Describe the “mud puppy” shown in Fig. 262. The pouched gopher, or rat
(Fig. 371), is sometimes absurdly called a salamander.
[Illustration:
FIG. 262.—BLIND SALAMANDER (_Proteus anguinus_). × ½. Found in caves
and underground streams in Balkans. Gills external, tail finlike,
legs small.
]