CHAPTER XIV
MAMMALS
Suggestions.—A tame rabbit, a house cat, or a pet squirrel may be taken to the school and observed by the class. Domestic animals may be observed at home and on the street. A study of the teeth will give a key to the life of the animal, and the teacher should collect a few mammalian skulls as opportunities offer. The pupils should be required to identify them by means of the chart of skulls (p. 194). If some enthusiastic students fond of anatomy should dissect small mammals, the specimens should be killed with chloroform, and the directions for dissection usual in laboratory works on this subject may be followed. There is a brief guide on page 223. The following outline for the study of a live mammal will apply almost as well to the rabbit or the squirrel as to the cat.
The Cat.—The house cat (Felis domestica) is probably descended from the Nubian cat (Felis maniculata, Fig. 333) found in Africa. The wild species is about half as large again as the domestic cat, grayish brown with darker stripes; the tail has dark rings. The lynx, or wild cat of America (Lynx rufus), is quite different. Compare the figures (333, 335) and state three obvious differences. To which American species is the house cat closer akin, the lynx (Fig. 335) or the ocelot (Fig. 334)? The domestic cat is found among all nations of the world. What is concluded, as to its nearest relatives, from the fact that the Indians had no cats when America was discovered? It was considered sacred by the ancient Egyptians, and after death its body was embalmed.
The body of the cat is very flexible. It may be divided into five regions, head, neck, trunk, tail, and limbs. Its eyes have the same parts as the eyes of other mammals. Which part of its eye is most peculiar? (Fig. 333.) What part is lacking that is present in birds? How are the eyes especially adapted for seeing at night? Does the pupil in the light extend up or down or across the iris? Does it ever become round?
Fig. 333.—Wild Cat of Africa (Felis maniculata), × ⅛.
What is the shape and the position of the ears? Are they large or small compared with those of most mammals? They are fitted best for catching sound from what direction? What is thus indicated in regard to the cat’s habits? (Compare with ears of rabbit.) Touch the whiskers of the cat. What result? Was it voluntary or involuntary motion? Are the nostrils relatively large or small compared with those of a cow? Of man?
Is the neck long or short? Animals that have long fore legs usually have what kind of neck? Those with short legs? Why? How many toes on a fore foot? Hind foot? Why is this arrangement better than the reverse? Some mammals are sole walkers (plantigrade), some are toe walkers (digitigrade). To which kind does the cat belong? Does it walk on the ends of the toes? Does it walk with all the joints of the toes on the ground? Where is the heel of the cat? (Fig. 334.) The wrist? To make sure of the location of the wrist, begin above: find the shoulder blade, the upper arm (one or two bones?), the lower arm (one or two bones?), the wrist, the palm, and the fingers (Fig. 337). Is the heel bone prominent or small?
Fig. 334.—Ocelot (Felis pardalis), of Texas and Mexico. × ⅑.
In what direction does the knee of the cat point? The heel? The elbow? The wrist? Compare the front and the hind leg in length; straightness; heaviness; number and position of toes; sharpness of the claws. What makes the dog’s claws duller than a cat’s? What differences in habit go with this? Judging from the toe that has become useless on the fore foot of the cat, which toe is lacking in the hind foot? Is it the cat’s thumb or little finger that does not touch the ground? (Fig. 337.) Locate on your own hand the parts corresponding to the pads on the forefoot of a cat. Of what use are soft pads on a cat’s foot?
Some animals have short, soft fur and long, coarse over hair. Does the cat have both? Is the cat’s fur soft or coarse? Does the fur have a colour near the skin different from that at the tip? Why is hair better suited as a covering for the cat than feathers would be? Scales? Where are long, stiff bristles found on the cat? Their length suggests that they would be of what use to a cat in going through narrow places? Why is it necessary for a cat to be noiseless in its movements?
Fig. 335.—Lynx (Lynx rufus). The “Bob-tailed cat” (North America).
Observe the movements of the cat.—Why cannot a cat come down a tall tree head foremost? Did you ever see a cat catch a bird? How does a cat approach its prey? Name a jumping insect that has long hind legs; an amphibian; several mammals (Figs. 362, 374). Does a cat ever trot? Gallop? Does a cat chase its prey? When does the cat move with its heel on the ground? The claws of a cat are withdrawn by means of a tendon (see Fig. 338). Does a cat seize its prey with its mouth or with its feet.?
How does a cat make the purring sound? (Do the lips move? The sides?) How does a cat drink? Do a cat and dog drink exactly the same way? Is the cat’s tongue rough or smooth? How is the tongue used in getting the flesh off close to the bone? Can a cat clean a bone entirely of meat?
Fig. 336.—Jaguar, of tropical America.
In what state of development is a newly born kitten? With what does the cat nourish its young? Name ten animals of various kinds whose young are similarly nourished. What is this class of animals called? Why does a cat bend its back when it is frightened or angry? Does a cat or a dog eat a greater variety of food? Which refuses to eat an animal found dead? Will either bury food for future use? Which is sometimes troublesome on account of digging holes in the garden? Explain this instinct. Which lived a solitary life when wild? Which had a definite haunt, or home? Why are dogs more sociable than cats? A dog is more devoted to his master. Why? A cat is more devoted to its home, and will return if carried away. Why? Why does a dog turn around before lying down? (Consider its original environment.)
Fig. 337.—Skeleton of Cat.
The Skeleton (Fig. 337).—Compare the spinal column of a cat in form and flexibility with the spinal column of a fish, a snake, and a bird.
The skull is joined to the spinal column by two knobs (or condyls), which fit into sockets in the first vertebra. Compare the jaws with those of a bird and a reptile. There is a prominent ridge in the temple to which the powerful chewing muscles are attached. There is also a ridge at the back of the head where the muscles which support the head are attached (Fig. 348).
Fig. 338.—Claw of Cat (1) retracted by ligament, and (2) drawn down by muscle attached to lower tendon.
Count the ribs. Are there more or fewer than in man? The breastbone is in a number of parts, joined, like the vertebræ, by cartilages. Compare it with a bird’s sternum; why the difference? The shoulder girdle, by which the front legs are attached to the trunk, is hardly to be called a girdle, as the collar bones (clavicles) are rudimentary. (They often escape notice during dissection, being hidden by muscles.) The shoulder blades, the other bones of this girdle, are large, but relatively not so broad toward the dorsal edge as human shoulder blades. The clavicles are tiny because they are useless. Why does the cat not need as movable a shoulder as a man? The pelvic, or hip girdle, to which the hind legs are attached, is a rigid girdle, completed above by the spinal column, to which it is immovably joined. Thus the powerful hind legs are joined to the most rigid portion of the trunk.
Mammals.—The cat belongs to the class Mammalia or mammals. The characteristics of the class are that the young are not hatched from eggs, but are born alive, and nourished with milk (hence have lips), and the skin is covered with hair. The milk glands are situated ventrally. The position of the class in the animal kingdom was shown when the cow was classified (p. 9). Their care for the young, their intelligence, and their ability to survive when in competition with other animals, causes the mammals to be considered the highest class in the animal kingdom.
According to these tests, what class of vertebrates should rank next to mammals? Compare the heart, the lungs, the blood, and the parental devotion of these two highest classes of animals.
Fig. 339.—Skeleton of Lion (cat family).
The first mammals, which were somewhat like small opossums, appeared millions of years ago, when the world was inhabited by giant reptiles. These reptiles occupied the water, the land, and the air, and their great strength and ferocity would have prevented the mammals from multiplying (for at first they were small and weak), but the mammals carried their young in a pouch until able to care for themselves, while the reptiles laid eggs and left them uncared for. The first mammals used reptilian eggs for food, though they could not contend with the great reptiles. Because birds and mammals are better parents than reptiles, they have conquered the earth, and the reptiles have been forced into subordination, and have become smaller and timid.
Fig. 340.—Walrus (Trichechus rosmarus).
Classification of Mammals.—Which two have the closest resemblances in the following lists: Horse, cow, deer. Why? Cat, cow, bear. Why? Monkey, man, sheep. Why? Rat, monkey, squirrel. Why? Giraffe, leopard, camel. Why? Walrus, cat, cow. Why? Check the five mammals in the following lists that form a group resembling one another most closely: Lion, bear, pig, dog, squirrel, cat, camel, tiger, man. State your reasons. Giraffe, leopard, deer, cow, rat, camel, hyena, horse, monkey. State reasons.
Fig. 341.—Weasel, in summer; in Canada in winter it is all white but tip of tail.
Fig. 342.—Foot of Bear (Plantigrade).
Teeth and toes are the basis for subdividing the class mammalia into orders. Although the breathing, the circulation, and the internal organs and processes are similar in all mammals, the external organs vary greatly because of the varying environments of different species. The internal structure enables us to place animals together which are essentially alike; e.g. the whale and man are both mammals, since they resemble in breathing, circulation, and multiplication of young. The external organs guide us in separating the class into orders. The teeth vary according to the food eaten. The feet vary according to use in obtaining food or escaping from enemies. This will explain the difference in the length of legs of lion and horse, and of the forms of the teeth in cat and cow. Make a careful study of the teeth and the limbs as shown in the figures and in all specimens accessible. Write out the dental formulas as indicated at the top of page 194. The numerals above the line show the number of upper teeth, those below the line show the number of lower teeth, in one half of the jaw. They are designated as follows: I, incisors; C, canine; M, molars. Multiplying by two gives the total number. Which skulls in the chart have the largest canines? Why? The smallest, or none at all? Why? Compare the molars of the cow, the hog, and the dog. Explain their differences. In which skulls are some of the molars lacking? Rudimentary? Why are the teeth that do not touch usually much smaller than those that do?
Fig. 343.—Polar Bear (Ursus maritimus).
| KEY, OR TABLE, FOR CLASSIFYING MAMMALS (class Mammalia) INTO ORDERS | ||||
|---|---|---|---|---|
| Orders | ||||
| A1 Imperfect Mammals, young hatched or prematurely born | ||||
| B1 Jaws a birdlike beak, egg-laying | Mon’otremes | |||
| B2 Jaws not beaklike, young carried in pouch | Marsu’pials | |||
| A2 Perfect Mammals, young not hatched, nor prematurely born | ||||
| B1 Digits with claws | C1 | Front part of both jaws lack teeth | Eden’tates | |
| C2 | Teeth with sharp points for piercing shells of insects | Insect’ivors | ||
| C3 | Canines very long, molars suited for tearing | Car’nivors | ||
| C4 | Canines lacking, incisors very large | Rodents | ||
| B2 Digits not distinct | C1 | Head large; carnivorous | Ceta’ceans | |
| C2 | Head small; herbivorous | Sire’neans | ||
| B3 Digits with nails or hoofs | C1 | Five toes, nose prolonged into a snout | Proboscid’eans | |
| C2 | Toes odd number, less than five | E’quines | Ungulates | |
| C3 | Toes even number, upper front teeth lacking, chew the cud | Ru’minants | ||
| C4 | Toes even number, upper front teeth present, not cud-chewers | Swine | ||
| C5 | All limbs having hands | Quad’rumans | ||
| C6 | Two limbs having hands | Bi’mans | ||
Exercise in Classification.—Copy the following list, and by reference to figures write the name of its order after each mammal:—
Use chart of skulls and Figs. 381, 382, 395–400 in working out this exercise.
Man’s dental formula is (M ⁵⁄₅, C ¹⁄₁, I ²⁄₂)2 = 32.
In like manner fill out formulas below:—
| Cow | (M—C—I—)2 = | 32 |
| Rabbit | (M—C—I—)2 = | 28 |
| Walrus | (M—C—I—)2 = | 34 |
| Bat | (M—C—I—)2 = | 34 |
| Cat | (M—C—I—)2 = | 30 |
| Armadillo | (M—C—I—)2 = | 28 |
| Horse | (M—C—I—)2 = | 40 |
| Whale | (M—C—I—)2 = | 0 |
| Am. Monkey | (M—C—I—)2 = | 36 |
| Sloth | (M—C—I—)2 = | 18 |
| Ant-eater | (M—C—I—)2 = | 0 |
| Dog | (M—C—I—)2 = | 42 |
| Hog | (M—C—I—)2 = | 44 |
| Sheep | (M—C—I—)2 = | 32 |
Fig. 344.—Skull and front of lower jaw of Cow.
Fig. 345.—Rabbit. A, B, incisors; C, molars
Fig. 346.—Walrus (see Fig. 341).
Fig. 347.—Bat.
Fig. 348.—Cat.
Fig. 349.—Armadillo.
Fig. 350.—Horse (front of jaw).
Fig. 351.—Greenland Whale.
Fig. 352.—American Monkey.
Fig. 353.—Sloth (Fig. 363).
Fig. 354.—Ant-eater (Fig. 364).
Fig. 355.—Horse.
Fig. 356.—Dog. Upper (A) and lower (B) jaw.
Fig. 357.—Hog.
Fig. 358.—Sheep.
Fig. 359.—Duckbill (Ornithorhynchus paradoxus).
The lowest order of mammals contains only two species, the duckbill and the porcupine ant-eater, both living in the Australian region. Do you judge that the duckbill of Tasmania (Fig. 359) lives chiefly in water or on land? Why? Is it probably active or slow in movement? It dabbles in mud and slime for worms and mussels, etc. How is it fitted for doing this? Which feet are markedly webbed? How far does the web extend? The web can be folded back when not in use. It lays two eggs in a nest of grass at the end of a burrow. Trace resemblances and differences between this animal and birds.
Fig. 360.—Spiny Ant-eater (Echidna aculeata). View of under surface to show pouch. (After Haacke.)
The porcupine ant-eater has numerous quill-like spines (Fig. 360) interspersed with its hairs. (Use?) Describe its claws. It has a long prehensile tongue. It rolls into a ball when attacked. Compare its jaws with a bird’s bill. It lays one egg, which is carried in a fold of the skin until hatched. Since it is pouched it could be classed with the pouched mammals (next order), but it is egg-laying. Suppose the two animals in this order did not nourish their young with milk after hatching, would they most resemble mammals, birds, or reptiles?
Write the name of this order. —— (See Table, p. 193.) Why do you place them in this order (——)? (See p. 193.) The name of the order comes from two Greek words meaning “one opening,” because the ducts from the bladder and egg glands unite with the large intestine and form a cloaca. What other classes of vertebrates are similar in this?
Fig. 361.—Opossum (Didelphys Virginianus).
Pouched Mammals.—These animals, like the last, are numerous in the Australian region, but are also found in South America, thus indicating that a bridge of land once connected the two regions. The opossum is the only species which has penetrated to North America (Fig. 361). Are its jaws slender or short? What kinship is thus suggested? As shown by its grinning, its lips are not well developed. Does this mean a low or a well-developed mammal? Where does it have a thumb? (Fig. 361.) Does the thumb have a nail? Is the tail hairy or bare? Why? Do you think it prefers the ground or the trees? State two reasons for your answer. It hides in a cave or bank or hollow tree all day, and seeks food at night. Can it run fast on the ground? It feigns death when captured, and watches for a chance for stealthy escape.
Fig. 362.—Giant Kangaroo.
The kangaroo (Fig. 362), like the opossum, gives birth to imperfectly developed young. (Kinship with what classes is thus indicated?) After birth, the young (about three fourths of an inch long) are carried in a ventral pouch and suckled for seven or eight months. They begin to reach down and nibble grass before leaving the pouch. Compare fore legs with hind legs, front half of body with hind half. Describe tail. What is it used for when kangaroo is at rest? In jumping, would it be useful for propelling and also for balancing the body? Describe hind and fore feet. Order ——. Why? ________. See key, page 193.
Imperfectly Toothed Mammals.—These animals live chiefly in South America (sloth, armadillo, giant ant-eater) and Africa (pangolin). The sloth (Fig. 363) eats leaves. Its movements are remarkably slow, and a vegetable growth resembling moss often gives its hair a green colour. (What advantage?) How many toes has it? How are its nails suited to its manner of living? Does it save exertion by hanging from the branches of trees instead of walking upon them?
Fig. 363.—Sloth of South America.
Fig. 365.—Nine-banded Armadillo of Texas and Mexico. (Dasypus novemcinctus.) It is increasing in numbers; it is very useful, as it digs up and destroys insects. (See Fig. 347.)
Judging from the figures (363, 364, 365), are the members of this order better suited for attack, active resistance, passive resistance, or concealment when contending with other animals? The ant-eater’s claws (Fig. 364) on the fore feet seem to be a hindrance in walking; for what are they useful? Why are its jaws so slender? What is probably the use of the enormous bushy tail? The nine-banded armadillo (Fig. 365) lives in Mexico and Texas. It is omnivorous. To escape its enemies, it burrows into the ground with surprising rapidity. If unable to escape when pursued, its hard, stout tail and head are turned under to protect the lower side of the body where there are no scales. The three-banded species (Fig. 366) lives in Argentina. Compare the ears and tail of the two species; give reasons for differences. Why are the eyes so small? The claws so large? Order______. Why? ______.
Fig. 366.—Three-banded Armadillo (Tolypeutes tricinctus).
Insect Eaters.—The soft interior and crusty covering of insects makes it unnecessary for animals that prey upon them to have flat-topped teeth for grinding them to powder, or long cusps for tearing them to pieces. The teeth of insect eaters, even the molars (Fig. 368), have many sharp tubercles, or points, for holding insects and piercing the crusty outer skeleton and reducing it to bits. As most insects dig in the ground or fly in the air, we are not surprised to learn that some insect-eating mammals (the bats) fly and others (the moles) burrow. Are the members of this order friends or competitors of man?
Fig. 367.—The Mole.
Fig. 368.—Skeleton of Mole. (Shoulder blade is turned upward.)
Why does the mole have very small eyes? Small ears? Compare the shape of the body of a mole and a rat. What difference? Why? Compare the front and the hind legs of a mole. Why are the hind legs so small and weak? Bearing in mind that the body must be arranged for digging and using narrow tunnels, study the skeleton (Fig. 368) in respect to the following: Bones of arm (length and shape), fingers, claws, shoulder bones, breastbone (why with ridge like a bird?), vertebræ (why are the first two so large?), skull (shape). There are no eye sockets, but there is a snout gristle; for the long, sensitive snout must serve in place of the small and almost useless eyes hidden deep in the fur. Is the fur sleek or rough? Why? Close or thin? It serves to keep the mole clean. The muscles of neck, breast, and shoulders are very strong. Why? The mole eats earthworms as well as insects. It injures plants by breaking and drying out their roots. Experiments show that the Western mole will eat moist grain, though it prefers insects. If a mole is caught, repeat the experiment, making a careful record of the food placed within its reach.
Fig. 369.—Skeleton of Bat.
As with the mole, the skeletal adaptations of the bat are most remarkable in the hand. How many fingers? (Fig. 369.) How many nails on the hand? Use of nail when at rest? When creeping? (Fig. 369.) Instead of feathers, the flying organs are made of a pair of extended folds of the skin supported by elongated bones, which form a framework like the ribs of an umbrella or a fan. How many digits are prolonged? Does the fold of the skin extend to the hind legs? The tail? Are the finger bones or the palm bones more prolonged to form the wing skeleton?
Fig. 370.—Vampire (Phyllostoma spectrum) of South America. × ⅙.
The skin of the wing is rich in blood vessels and nerves, and serves, by its sensitiveness to the slightest current of air, to guide the bat in the thickest darkness. Would you judge that the bat has sharp sight? Acute hearing?
The moles do not hibernate; the bats do. Give the reason for the difference. If bats are aroused out of a trance-like condition in winter, they may die of starvation. Why? The mother bat carries the young about with her, since, unlike birds, she has no nest. How are the young nourished? Order ________. Why? ________. (Key, p. 193.)
Fig. 371.—Pouched Gopher (Geomys bursarius) × ¼, a large, burrowing field rat, with cheek pouches for carrying grain.
Fig. 372.—Hind foot a, fore foot b, tail c, of Beaver.
Fig. 373.—Beaver.
The Gnawing Mammals.—These animals form the most numerous order of mammals. They lack canine teeth. Inference? The incisors are four in number in all species except the rabbits, which have six (see Fig. 345). They are readily recognized by their large incisors. These teeth grow throughout life, and if they are not constantly worn away by gnawing upon hard food, they become inconveniently long, and may prevent closing of the mouth and cause starvation. The hard enamel is all on the front surface, the dentine in the rear being softer; hence the incisors sharpen themselves by use to a chisel-like edge. The molars are set close together and have their upper surfaces level with each other. The ridges on them run crosswise so as to form a continuous filelike surface for reducing the food still finer after it has been gnawed off (Fig. 345). The lower jaw fits into grooves in place of sockets. This allows the jaw to work back and forth instead of sidewise. The rabbits and some squirrels have a hare lip; i.e. the upper lip is split. What advantage is this in eating? In England the species that burrow are called rabbits; those that do not are called hares.