B. Studies of Amphibia

Progress from Water-living Animals to Land-living Animals

The Living Frog or Toad

To show how an Animal may be adapted to both Land and Water Life

Materials.

Living frogs in small cages or aquaria; living toads; some pungent liquid, as ammonia.

Observations.

The Body.

Contrast the body of the frog with that of the fish as to regions, shape, and compactness. How do you account for the differences?

What kinds of locomotion can a frog use? Why is it difficult for a frog to crawl or walk? How far can a frog jump? How are swimming and jumping accomplished? What do you think is the use of the "hump" on the back?
Identify in the legs the thigh, shank, ankle, foot, toes, and web, and in the arm, the upper arm, forearm, wrist, hand, and fingers. State in detail the differences in structure and in position between the fore and hind limbs. How do you account for these differences?

Induce a frog or toad to eat by dangling food, such as a piece of raw meat or meal worms, small earthworms, etc., before it. How does it seize the food? What will it eat? How is the mouth adapted to this manner of feeding?

The frog has no diaphragm, and therefore no chest cavity; watch very carefully the movements of the mouth, the nostrils, the throat, and the sides of the body to determine how the problem of breathing (how the air is gotten into and out of the lungs) without a diaphragm is solved. Write a detailed account of the frog's method of breathing which shall explain just how the air is forced into the lungs.
What would be the effect of propping open the mouth of the frog? Why? Does the frog breathe in the usual manner while under the water? If not, how do you explain its ability to remain under water for a long period of time?

Investigate the efficiency of the five special senses in the frog by devising experiments to test each sense; as, for example, giving a frog its liberty on the floor and trying to catch it again, to test the sense of sight. Write an account of your experiments and their results. Which of the senses is best developed? Give reasons for your answer.
Compare the eye of the frog with respect to its shape, movements, parts, and protective structures with that of the fish. In what respects are they similar? in what respects different? Why should they differ?
Where are the frogs' ears located? What do you think of the efficiency of an eardrum situated on the surface of the body? Why?
The frog has certain other responses. Try turning the jar or cage containing a frog around to face the frog in another direction. What happens? How do you explain this response on the basis of use? What other responses have you noticed?

Has the frog an exoskeleton? Describe the color scheme of the frog and explain how it may be protective. Why are frogs brighter in spring than in fall?
Why do frogs usually live near water? Do they ever leave the vicinity of streams? If so, when?

Summary.

In what ways is the frog adapted to water life? In what ways to life on land? In what respects do toads differ from frogs?

The Frog's Mouth

Materials.

Preserved or freshly killed frogs in dishes or shallow pans of water; forceps and a bristle.

Observations.

Open the frog's mouth as widely as possible and, if necessary, insert a splinter to hold the jaws apart. Identify the following structures:—

Tongue. Draw it forward until the free end extends from the mouth and is outspread; observe its form, extent, and attachment. Teeth. Find those on the jaws and on the roof of the mouth. Nostrils. Push the bristle inward through a nostril to determine its direction and extent. Vocal cords. These form a hard white mass in the floor of the mouth, well back behind the tongue. Glottis, the slit inclosed [between] the vocal cords, opening into the trachea. Esophagus, the passage to the stomach, at the posterior end of the mouth. Eustachian tubes, small passages outward to the ears at the junction of the upper and lower jaws.

Questions.

Describe the probable action of the frog's tongue in catching a bug.
What advantage can you ascribe to the peculiar mode of attachment of the tongue?
Of what use is the notch in the inner end of the tongue? (Note its position when the tongue lies at rest in the mouth.)
If the frog chewed its food, how would the existing structure of the nostrils be very inconvenient?
Recall either the frog's habit of feeding or the structure of the nostrils. Do you think the nostrils are of much service in smelling? State the reason for your answer.
Of what use are the vocal cords and why are they so muscular? Consider their use in sound making and also their condition during swallowing.
Of what use are the teeth? Recall the form and use of the fish's teeth.

Suggested drawings.

The mouth, wide open and with tongue extended.
A diagram showing the path of air and of food through the frog's mouth.

The Organs of Digestion, Absorption, and Excretion

Materials.

Freshly killed or preserved frogs in dishes or shallow pans of water, forceps, and, if the pupil is to do any dissecting, scissors.

Directions and Observations.

The specimens may have been opened by the teacher, or may be dissected by the pupil as follows:—

Placing the frog on its back, with forceps firmly grasp the skin of the abdomen and the muscles beneath, just in front of the hind legs, and with the scissors cut straight forward in the middle line until the floor of the mouth is reached; this will separate the arms. Care must be taken not to cut too deeply, but this may be avoided by keeping the skin uplifted. Now cut sidewise in front of each hind leg in order that the body wall may be laid aside. Under the arms the heart will be seen; it will be studied as a part of the circulatory system.

Identify the following organs:—

Liver, the large red or brown mass, consisting of several divisions and lying close up under the arms. Bile sac, small, green, and between the liver lobes. Alimentary canal.

Mouth.
Esophagus.
Stomach, the elongated, light-colored, firm, and muscular portion.
Small intestine, a slender, more or less closely coiled, tubular portion.
Large intestine, a thin-walled enlargement at the posterior end of the canal.
Duodenum. This is a muscular portion of the small intestine immediately following the stomach, against which it is folded.

Note.—Specimens secured in late fall, winter, or early spring may contain, if female, a large number of dark-colored eggs; or if male, two white testes, located near the kidneys and similar to them in form, though smaller.

Questions.

Name the parts of the frog's alimentary canal.
Name the glands or organs which are accessory to the canal.
How long is the esophagus? How does the presence or absence of a neck affect the esophagus?
How does the thickness of the stomach wall compare with that of the intestine, and how do you account for the difference?
Measure the length of the trunk of the frog's body and that of the outstretched alimentary canal. How many times the length of the one is that of the other? How does this ratio compare with that of an herbivorous animal? (The sheep's food canal is about thirty-two times the length of its body.)
What is the color of the bile, as seen through the walls of the bile sac? This color is characteristic of carnivorous animals; in herbivorous forms it is yellow. Find its color in some omnivorous form, as man.
Name the organs concerned in excretion.
What holds the internal organs in place, and from what are they suspended?
The spleen is called a "ductless gland." Give its function, and explain why a duct is not necessary to it.
Since the frog swallows its food alive and entire, what work must the stomach do? What digestive organs would be absent from the mouth, or else poorly developed?
Since the frog is carnivorous, what digestive ferments are probably present, and what ones absent from the alimentary canal?
Fat bodies are largest in the fall, and are rarely found in the spring. How can you account for this?
When through with the general study of the alimentary canal, you may open the stomach by cutting it lengthwise. Describe the character of the stomach lining as to folds and villi, stating the advantage of each being present and the reason for the direction of the folds. Tell how the food is propelled onward through the alimentary canal. Give the scientific name for this action.

Suggested drawings.

The viscera (internal organs) undisturbed.
The alimentary canal extended.
The excretory system.

The Organs of Circulation and Respiration of the Frog

Materials.

The materials used in this exercise are the same as those used in the preceding exercise.

Observations.

The pupil should identify the following structures:—

Pericardium, a membrane that surrounds the heart and, in the case of the frog, separates the body cavity into two portions, the abdomen and the pericardial chamber. Heart, lying between the shoulders and in front of the false diaphragm. It is conical in form and composed of three chambers. Ventricle, the pink, conical, and muscular portion of the heart, pointing backward and outward. Auricles, right and left. These are anterior and dorsal to the ventricle, thin, membranous, and dark-colored. Arterial trunk, the single large blood vessel, usually empty of blood, and white. Note its origin and trace it as far as possible, at least until you see it divide to encircle the throat. Sinus venosus, a large membranous sac dorsal to the heart and connected with the right auricle. Lungs, two small oblong, pink, spongy sacs, lying between and behind the shoulders.

The pupil may also identify the following structures if a specimen is available which has the blood vessels injected.

Conus arteriosus, or "arterial trunk," a large artery passing obliquely forward from the ventricle, and dividing into three branches on each side. Carotid arteries, the first branches of the conus, to the head and neck. Aortas, the second branches of the conus, to the dorsal region. Pulmocutaneous arteries, the third branches of the conus, to the lungs and skin. Dorsal aorta, the large artery along the back, formed by the union of the two aortas. Iliac, or femoral arteries, the two posterior divisions of the dorsal aorta, supplying the legs. Posterior (ascending) vena cava, a large vein close to the dorsal aorta, passing forward from the kidneys. Hepatic veins, large veins connecting the liver with the posterior vena cava. Anterior (descending) venæ cavæ, large veins formed by the junction of the veins from the arm, neck, and head on the right and left sides. Subclavian veins, from the arms. Jugular veins, from the neck.

Questions.

Of how many chambers does the frog's heart consist? Name them, and describe them as to size, color, and structure.
Which chamber receives blood from the body, and which receives blood from the lungs?
Name the large arteries and give the regions which they supply. Name the large veins and give the regions from which they come.
Describe the lungs as to size, both when inflated and when uninflated. Describe their color; entirety or subdivision; texture.
In the inflated lung, notice the interior partitions or chambers, which are called vesicles. How do they affect the amount of surface exposed for gas exchange in breathing?
Measuring the lung collapsed and again when inflated, calculate its approximate volume in each case and state how much air it may take in during an inspiration.
Describe the diaphragm and state its probable use as a factor in respiration or as a partition.

Advanced questions.

Apparently the pure and the impure blood must commingle upon entering the single ventricle, but by a simple device this scarcely occurs. How would such commingling affect the purity of the blood as it reached the tissues, and hence affect waste removal, oxidation, body temperature, activity, and intelligence?
What are the chambers of the fish's heart? of the mammalian (human) heart? How may the heart of the frog be regarded as intermediate between these others? How and where might the growth of a partition within it bring about the higher structure? (This actually occurs in certain reptiles.)
The lungs are said to be outgrowths of the alimentary canal. Explain how their connection would tend to show this.

Suggested drawings.

The heart and lungs in their normal position.
The circulatory system, as seen in the injected specimen.
A copy of the diagram or model of the circulatory system.
A diagram of the respiratory tract from the nostrils to the lungs, by arrows showing the course of the air.

The Nervous System of the Frog

Materials.

Specimens which have the brain exposed and other specimens whose viscera have been removed so that the spinal nerves can be seen; pans or shallow dishes of water and forceps.

Observations.

The nerve tissues are generally white in color unless they have been specially treated and stained. The pupil should identify the following structures:—

A. Brain, those enlargements of nerve tissue situated in the head and composed of four principal parts, as follows:—

Cerebral hemispheres, a pair of elongated lobes, the anterior enlargements.
Mid brain, or optic lobes, a pair of large ovoid structures, projecting diagonally forward and sidewise.
Cerebellum, a slender, transverse ridge, close behind the midbrain.
Medulla, the anterior end of the spinal cord, widest in front and containing a triangular depression.

(Frequently a pair of smaller enlargements is to be seen in front of the cerebral hemispheres; they are the olfactory lobes, and from them nerves pass forward to the nasal chamber.)

B. Spinal cord, extending along the spine, giving rise to nerves.

C. Spinal nerves, ten pairs of nerves which are connected with the cord through dorsal (sensory) and ventral (motor) roots, and which penetrate the body and its appendages. The first enters the neck; the second and third join and enter the arm; the fourth to sixth penetrate the skin and muscles of the trunk; the seventh to ninth join by a plexus to form the sciatic nerve which supplies the leg, and the tenth enters the posterior portion of the body.

Questions.

Which lobes of the brain are paired? Give at least two possible causes or reasons for their double structure.
The optic lobes are connected with the eyes. Compare their size with that of the other parts. Of how much use do you think they are to the frog?
If folds or convolutions in the surface of the cerebrum indicate intelligence, thoughtfulness, or mind, what do you infer as to the frog's mental condition and power to think?
Has the frog brain or "brains"? Explain.
Name several things done by an animal's brain.
Which of the spinal nerves are specially large? Why should they be large?
What advantages are there in the frog's having a dorsal nerve cord instead of a ventral one, as the earthworm has?
What would be the effect of cutting or breaking the dorsal root of a spinal nerve? the ventral root? the entire spinal cord? How do you explain the convulsions of the brainless (beheaded) chicken or frog?
What kinds of impulses originate outside of the nerve center, and what kinds in the center?
Sensations reach the cord and brain through the dorsal root of the spinal nerve. What kinds of messages travel through the ventral root, and in what direction do they go?

Suggested drawings.

The brain.
The spinal cord and its nerves.
The nervous system.
A diagram of the cord and its nerves, showing the kinds and the directions of the nerve impulses.

The Endoskeleton of the Frog

Materials.

Prepared frog skeletons mounted in glass-covered boxes or in other cases suitable for individual study; other vertebrate skeletons for reference.

Observations.

The pupil should examine his specimen and identify the following structures:—

A. Skull:—

Cranium, or brain case, the central and hinder portion.
Nasal bone, a triangular bone lying in front of each large opening, or eye orbit, and attached to the anterior end of the cranium.
Premaxillaries, a pair of small bones which form the tip of the nose.
Maxillary, a slender bone forming the side of the upper jaw.
Dentary, the bone of the lower jaw, corresponding to the maxillary.
Occipital foramen, the posterior opening or entrance into the cranium, normally covered dorsally by cartilage and most easily seen in a separate skull.

B. Vertebral Column:—

Cervical vertebra, the first vertebra, supporting the skull.
Dorso-lumbar vertebræ, vertebræ with small lateral processes.
Sacrum, the ninth vertebra, bearing extra long lateral processes or "arms."
Urostyle, the last vertebra, lying in the median line, a long, slender, blade-like bone, really formed by the fusion of several vertebræ.

C. Vertebra:—

Centrum, the solid, nearly circular portion.
Lateral process, one of the paired projections, extending outward on either side.
Neural spine, the single projection, extending toward the dorsal side.
Neural arch, formed largely by a connection between the lateral process and the neural spine. The arch above and the centrum below inclose the neural canal.

D. Pectoral, or Shoulder, Girdle:—

Sternum, the "breastbone," extending along the median ventral line.
Coracoid, a heavy bone extending from the sternum sidewise to support the arm.
Clavicle, the "collar bone," a light bone in front of the coracoid.
Scapula, the "shoulder blade," a broad, thin bone which arches around to the dorsal side.

E. Pelvic, or Hip, Girdle:—

Ilium, a long, slender, curved bone, with its mate uniting to form an inverted "wishbone"; it is joined to the sacrum.
Pelvis, the region of the socket at the junction of the ilia.

F. Fore Leg, or Arm:—

Humerus, the single bone of the upper arm.
Radius, the bone of the lower arm on the thumb side; in the frog united to the ulna.
Ulna, the bone of the forearm opposite the radius.
Carpals, small bones of the wrist.
Metacarpals, a single series of long bones forming the palm of the hand.
Phalanges, the bones of the fingers and thumb (singular phalanx).

G. Hind Leg:—

Femur, the thigh bone, next to the body.
Tibia, the larger bone of the lower leg on the inner side; in the frog united to the fibula.
Fibula, the smaller bone of the lower leg.
Tarsals, the small bones of the instep.
Metatarsals, the long bones of the instep.
Phalanges, the bones of the toes.

Questions.

Whereabouts in the frog has nature made an attempt to inclose delicate or vital structures in bony cases?
Examining the inside of the mouth, find the teeth. Judging from their size and structure, of what use are these teeth?
What advantages can you see in having the arms and legs attached to girdles instead of having them fastened directly to the vertebral column? How has their development affected the shape of the trunk, as opposed to that of the fish?
Make a comparison of the two girdles as to their attachment to the spine and their consequent rigidity or freedom of movement.
How many vertebræ are there in the spinal column? What advantage can you see in having the column composed of many small vertebræ instead of a few large ones? Enumerate those having a special form or structure, and state the use of each.
On the vertebræ notice any irregularities, prominences, or roughenings. For what are such bones better adapted than smooth bones would be?
How is the frog's humping permitted? How are the urostyle and the pelvis connected, if at all? Has this any effect on motion? (See living frog.)
In parallel columns, keeping corresponding parts in a line, tabulate the bones of the fore and hind limbs.
State how extra length has been attained in the hind leg, and give the purpose or the result of this lengthening.
In the forearm notice the fusing of the radius and ulna. How would these parts act in rotating the hand, as compared with your own, where they are free? Compare also the tibia and fibula. Where in the frog's leg is turning made possible by the use of parallel bones?
Compare the frog's hand and foot as to number of fingers and toes; as to length and spread. Have any fingers or toes only two phalanges?
How would an inner skeleton affect the growth and the size of an animal, as compared with an outer skeleton, like the clam's?
How would the lack of an outer skeleton influence sensitiveness, activity, and intelligence?

Suggested drawings.

The skull and trunk, with appendages of one side.
Each set of bones separately, as the skull, the column, etc.

Comparative Study of Amphibia

Materials.

Various amphibia, either dead or alive, such as newts, mud puppies (necturus), salamanders, and several species of frogs and toads, especially tree toads.

Observations.

Answer the following questions with respect to each animal:—

What regions of the body are present? What is the general shape and size of the body?
For what kind of locomotion are the limbs fitted? How?
Have the hands and feet any special adaptations? If so, what are they and for what purpose?
With what organs does the animal breathe? If with gills, are they external or internal?
Judging from the specimen, what do you think is the habitat? Give reason for your answer.

Summary from the Comparative Study of Amphibia

Which of the amphibia in this study are fish-like in character? What are the fish-like characters? Do you think these fish-like amphibia are of a lower or higher type than the others? Give reasons for your answer.
Show how the variation in (a) the form of the body, (b) color patterns, and (c) the structure of the legs and arms of the amphibia are related to habitat and mode of life.

Amphibia: A General Review and Library Exercise

The usefulness of the American toad.
The breeding habits of the common frog; of the Surinam toad; of the obstetrical toad; of the "smith."
The development of the axolotl.
The habits of the tree frogs and their variable coloration.
The croaking of frogs and of toads.
The flying frog of Ceylon.
The distribution and habits of the hellbender; of the mud puppy; of the Congo "snake."
The general absence of the amphibia from arid regions.
Frog farming for city markets.
Protective devices of various amphibia.
Toads and warts.
The homing and water instincts of toads.
Hibernation, seasonal or prolonged, of toads and frogs.
The changes in the respiratory and circulatory system during metamorphosis.
Ontogeny and phylogeny, as illustrated by amphibia.
The structural defects which would prevent an ambitious frog from becoming a highly trained and skillful animal.
The classification of amphibia, with examples.
The kinds of frogs and toads found in the region where you live.