GILL CLEFTS AND HEART

though dissociated from the respiratory function, remains for life. That cleft is the external ear hole, which opens internally by what is called the eustachian tube into the pharynx. Thus, though the gill clefts in the higher vertebrates are no longer used for breathing purposes, being replaced by the lungs, they persist for a time to emphasize the relationship between the higher and the lower vertebrates.

The ventrally placed heart is another marked characteristic of the vertebrata. The beat of the heart in man is felt through the chest walls and not through the back. In all animals the heart has the same position; it lies on the opposite side of the body to that which lodges the brain and spinal cord. In many invertebrate animals the position of these two important organs is exactly the reverse. In worms, insects and crustaceans, the heart, if it exists, is on the dorsal side and the larger portion at any rate of the central nervous system is on the ventral side, the side upon which the animal progresses, or which is turned towards the ground in progression as in those animals which possess legs.

Anatomy reveals many other characteristics of vertebrate animals; but the principal ones are those which we have just briefly sketched. The large assemblage of creatures classifiable as vertebrates splits up readily into the five sub-groups mentioned above, all of which are considered in the following pages.

INAC

NACCURATE though the term undoubtedly is (for all "mammals" do not

tht possess mammæ, or teats)

66

in application from derivation, it is better to adhere to the expression in lieu of something better, than to use or revert to the entirely wrong vernacular words in common use, viz., "Quadrupeds," Beasts," or Animals." For lizards are quadrupeds, and the converse of animal is clearly vegetable or mineral. "Beasts" might be retained, but that it has a somewhat insulting suggestion. The justification of the term, however, is less important than its meaning. What are we to understand by mammals or mammalia? For us who are concerned here only with living mammals, the distinction from other backboned animals is quite easy and obvious, even without having recourse to bony and other structural characters. By external form and character only, if rightly put, it is within the power of any one to recognize clearly the features which enable him to assert positively that a given animal is or is not a mammal. It is, in the first place, not correct to define mammals as quadrupedal; most of them are so indeed, and all that especially concern us in this place. But if we attempt to make definitions that suit the entire "class," it is requisite to allow for these exceptions. To begin with, man is not

MAMMALS AND LIZARDS

quadrupedal except occasionally and during his young stages; the whales are clearly not so; neither are the manatees and dugongs; the bats, it is true, do shuffle along on their "wings" as well as by the aid of their hind limbs; but it would be straining the proper use of the term to describe them as "quadrupeds." But when quadrupedal, as in the vast majority of mammals, the fore and hind limbs lie beneath and actually support the body more directly than in the other quadrupedal group, the Reptiles. In the latter it will be noticed. that the body is, as it were, slung between the legs like the body of an eighteenth-century coach between its wheels ; in the mammals the legs support the body as its legs do a chair. We may, however, legitimately say that the mammalia are much more generally quadrupedal than are the reptiles, of which (see below) so many are entirely or partially legless. The next point is one which absolutely distinguishes all mammals from all other vertebrates (and, of course, invertebrates). This feature is the covering of hair. To this statement there are no real exceptions, but several apparent exceptions, which we must note. The apparent exceptions, again, are of animals which really do not enter into the subject matter of this volume. A very little observation will convince any one that the almost naked rhinoceros has some vestiges of a hairy covering. But the whales, again, which contradict so many generalizations about the class mammalia, are rather more deceptive. Their covering of hair is reduced to a few hairs in the neighbourhood of the mouth. If these were to go it would be most difficult to state definitely, from external characters only, that a whale was not an aquatic reptile like the Ichthyosaurus. There are, moreover, a series of exceptions on the other side. Some of the feathers of flightless birds, such as the Apteryx, have to the naked eye every appearance of hairs. Their microscopic character, and

EXTERNAL EARS

the way in which they develop in the embryo, can alone settle the fact that they are not hairs, but true, though somewhat rudimentary, feathers. But no practical difficulty arises in this case; for on other parts of the body are plenty of obvious feathers which no mammal ever possesses.

A third feature is also absolutely distinctive, and that is the presence in the female, with rudiments in the male, of cutaneous glands which secrete milk for the nourishment of the young when born. No other vertebrate possesses anything even remotely resembling the mammary glands of the mammalia.

A rash observer might say that mammals are never scaly, while reptiles, fishes, and even birds (their feet) are always so. It is difficult, however, to distinguish accurately the scales upon a rat's tail from those of a lizard, though it is true that the scales of the pangolin, or scaly ant-eater of Africa and India, are not real scales, but merely agglutinated hairs.

No one can look at the head of a mammal, that is of course of a terrestrial mammal (for here again, as in so many features, the whales are exceptional), without observing how impossible it is to mistake that group for any of the lower lying groups of vertebrate animals. With a few exceptions-such as the seals as well as the whales already mentioned and a small selection of other mammals—external ears are present in that group, and often of conspicuous size. In the lizards, frogs and birds the external ear is absent, and there is merely the external auditory passage visible, covered by the tympanic membrane; and even that is not always present. Coupled with the presence of ears is a look of alertness, quite distinctive of the mammalia, and a more fidgetty demeanour than that of lower vertebrates, except of course birds. There is but little torpidity among mammals save in those few cases, such as the hedeghog, where the animal

COLOURS OF MAMMALS

hibernates. They do not remain for hours in a motionless condition save when asleep.

So far as external characters are concerned the above observations embody the principal points in which the group differs from those of vertebrates lying lower in the series. There are, however, still remaining, a few characters which are highly noteworthy among the mammalia without being absolutely diagnostic. It will be readily observed in reviewing the large series of mammals at the Zoo, and comparing them with the larger series of birds, that the mammalia for the most part are clad in a sober livery. There is nowhere in the group that development of brilliant primary colours such as we see so very commonly among birds. There are no reds, greens and blues, the utmost brilliancy of hue being bright browns contrasting with white, and a few brighter colours, not of hair, but of naked skin, such as the muzzle and ischial callosities of some monkeys. If may be that correlated with this absence of striking coloration is the predominance of the senses of smell and hearing over sight. It is broadly true of the mammalia, that they smell and hear rather than see. While of birds it is equally true to say, that they see rather than smell. That this point of view is in the main correct is shown by another very characteristic feature of the mammalia. In various parts of the body glands open on to the surface; these glands produce variously smelling secretions, of which musk, civet, and castoreum, are examples. The odoriferousness of the houses in which the mammalia are confined is an unmistakable proof of this; and the fact that birds are at least by no means so strongly smelling convinces. us of the absence of glands in the skins of those animals. The same may be said of reptiles and amphibia, broadly considered-for here as elsewhere there are exceptions. There are for example musky smelling glands among