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?
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].)
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?
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.
Fig. 253.—Skeleton of Frog.
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.
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?
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.
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.
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.
Fig. 259.—Brain of Frog.
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]).
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.
Fig. 262.—Blind Salamander (Proteus anguinus). × ½. Found in caves and underground streams in Balkans. Gills external, tail finlike, legs small.
CHAPTER XII
REPTILIA (REPTILES)
This class is divided into four orders which have such marked differences of external form that there is no difficulty in distinguishing them. These orders are represented by Lizards, Snakes, Turtles, and Alligators. Of these, only the forms of lizards and alligators have similar proportions, but there is a marked difference in their size, lizards being, in general, the smallest, and alligators the largest of the reptiles.
Fig. 263.—A Salamander.
Fig. 264.—A Lizard.
Comparison of Lizards and Salamanders.—To make clear the difference between reptiles and batrachians, it will be well to compare the orders in the two classes which resemble each other in size and shape; namely, lizards and salamanders (Figs. [263] and [264]). State in a tabular form their differences in skin, toe, manner of breathing, development from egg, shape of tail, habitat, habits. Each has an elongated body, two pairs of limbs, and a long tail, yet they are easily distinguished. Are the differences suggested above valid for the other batrachians (frogs) and other reptiles (e.g. turtles)? Trace the same differences between the toad or frog (Fig. [250]) and the “horned toad,” which is a lizard (Fig. [265]).
Fig. 265.—“Horned Toad” Lizard, of the Southwest (Phrynosoma cornita). × ⅔.