CHARACTERS OF CROCODILES

to bask upon the shore, or to make short excursions in quest of other pools, is not a feature which absolutely differentiates the group from the lizards, for there are many aquatic forms to be found there; but it is a feature which is characteristic of all crocodilians. These reptiles do not follow the lacertilian plan of shedding their skin in conspicuous bits; the wear and tear of the outer covering of the body is like that in ourselves; it is constant and imperceptible. Underneath the scales are bony plates, which are mostly to be found only upon the back, though in the caimans of South America there are ventral scutes, as these armoured plates are termed, in addition to those upon the back. No lizard has so extensive a series of plates of this kind, though there are corresponding, but only slightly developed, plates in some forms. Many of these plates, and the bones of the head also, are irregularly pitted and honeycombed, a characteristic which can be readily seen in crocodiles. The nostrils, it will be noticed, are at the top of the snout, and the animal can lie nearly submerged, with only the nostrils and eyes projecting. An ingenious apparatus, not found in lizards, enables crocodiles and alligators to swallow their prey beneath the surface without running the risk of choking themselves. The internal nostrils open into the throat far back, and a soft downhanging process shuts them off from the proper mouth cavity, which can be thus opened and closed without admitting water into the lungs. In internal anatomy the differences which distinguish this group of reptiles are numerous and profound.

Although the Chinese alligator was not known to Europe forty years ago, the Chinese writers were well acquainted with it, and heaped it round with much legend and mystery. The N'Go or To, as this animal is named, is said to attain to an extremely green old

Z.G,

273

T

AGE OF CHINESE ALLIGATOR

age; indeed, its capacity for longevity has given rise to a proverbial expression comparable to Methusaleh or Old Parr among ourselves. The Chinese zoologists, or curiosity-mongers, to use a more descriptive term, variously termed the N'Go a fish, a dragon, and a tortoise. They prized it in medicine, and Marco Polo himself was gravely of opinion that its merits were high; the gall he recommended as a cure for the bite of a mad dog. But its usefulness is by no means confined to this disease, for, like certain much-advertised drugs among ourselves, there appears to be no ill that Chinese flesh is heir to that the carcass of this beast will not furnish remedies for. As alligators and crocodiles go, this Chinese representative of the family is not large; it is a dwarf beside the twenty-foot long crocodiles, and still more so when compared with the huge extinct Siwalik crocodilian, which measured fifty feet from snout to end of tail. Of other crocodiles and alligators plenty of examples are certain to be in the Reptile House.

POSITION OF AMPHIBIA

THE

CHAPTER IX

Amphibia and Fishes

HE amphibia, those cold-blooded and reptile-like creatures, really stand midway between the reptiles proper and the underlying fishes. A study even of those characters which can be seen without recourse to the scalpel help in fixing pretty definitely the place of the amphibia in the scheme of Nature. It will be convenient to emphasize the relationships and differences between the amphibia and the reptiles by taking two representative types, one of each, and then comparing them character by character. Later the results can be amended by the consideration of exceptions, so as to apply to the whole group in either case. We may select for our comparison the North American Menobranchus, of which there are practically certain to be examples in the Reptile House, and any lizard among the large assortment contained in the same house; let us say either of the common British species, Lacerta vivipara or agilis. The newt-like amphibian is purely aquatic, while the lizard is as eminently terrestrial, selecting indeed especially the driest of sandy localities. The Menobranchus loves coolness and darkness; the lizard rejoices in the day and in warm sunlight. The skin of the menobranch is soft and slimy; not only are there no scales of any kind, but the epidermis is everywhere developed into slime-producing glands, which cover the body with their secretions. In the

REPTILES AND AMPHIBIANS

lizard the body is dry and hard, the epidermis being converted into regularly arranged scales, while there are no glands in the skin except at the beginning of the thighs, where a row of pores void on to the exterior the secretion of the femoral glands. The limbs of the amphibian are short and only four-toed, rot much used in locomotion. Those of the lizard are five-toed and longer, and very much used in locomotion. On the neck of the menobranch will be noticed three pairs of fringed red outgrowths of the body, which are the gills; by means of these the animal at least partially breathes. It has also lungs. Close to the gills are, on each side of the neck, two perforations which lead into the pharynx; these are the so-called gill clefts. In the lizards we find not the slightest trace of these last two structures. The animal has only lungs, with which alone it breathes. If we dive into the anatomy of the two animals we shall emerge with other characters, which completely distinguish them. The skull of the Menobranchus is fixed on to the vertebral column by two joints or condyles while there is but one median condyle in the lizard. The heart is threechambered in both; but in the amphibian the origin of the aorta from the ventricle is dilated into a thicker walled tube, in which are several series of valves regulating the flow of the blood when it leaves the heart to pass through the arterial system. In the lizard there is no such dilated conus arteriosus," and the persistence of this conus in the amphibian is a mark of its relationship to the lower lying fish, where such a conus always exists. The menobranch has no sternum or breast-bone formed by the union of the ribs ventrally; there is such a sternum in the Lacerta. The Menobranchus lays eggs as does the Lacerta; but the eggs of the former are smaller than those of the latter, and do not contain nearly so much yolk. Moreover there is no hard shell such as