This is one of the most singular of all the amphibians. It is found in North America. Sometimes it develops into its perfect form, and sometimes it remains a tadpole all its life, and yet lays eggs just as though it were adult! In the lakes of the southern Rocky Mountains the life of this creature is just like that of any other batrachian. That is, it is hatched out of the egg as a tadpole, grows first one pair of legs and then another, loses its gills by degrees, and at last appears in a lizard-like form, leaving the water and living upon dry land. But in the lake which surrounds the city of Mexico it never becomes anything more than a big tadpole, keeps its gills throughout its life, and does not leave the water at all. THE OLM The olm, or proteus, is found only in the underground lakes of Carniola and one or two other parts of Central Europe. It is about a foot long when fully grown, and has a slender, snake-like body, with a pair of tiny legs just behind the head, and another pair at the base of the tail. It is perfectly blind, the eyes being hidden under the skin, and yet cannot bear light. For if it is kept in captivity it will always hide in the darkest corner that it can find. And it has been known to live in confinement for five years without once taking any food. What the habits of this extraordinary animal are in nature no one knows, as it has never been found except in these underground lakes. In color the olm is pinkish gray, with bright-red gills, and there are from twenty-four to twenty-seven grooves upon either side of its body. FISHES CHAPTER XXX FRESH-WATER FISHES The lowest class of the vertebrate animals consists of the fishes. These are easily distinguished. Some of the reptiles, it is true, are very fish-like. But then they have three chambers in their hearts, while the true fishes only have two. Then fishes never have limbs, the place of which is taken by fins; and further, they breathe water by means of gills. There are other differences as well; but these are quite sufficient to show us that reptiles and fishes cannot possibly be mistaken for one another. Between the two, however, come several very curious creatures, which seem to be partly reptiles and partly fishes; for they have four slender members which hardly seem to be legs, though they cannot possibly be described as fins, while they possess not only gills but lungs as well. THE MUD-FISH One of these is the odd mud-fish of the African rivers. In general appearance this animal looks something like an eel, and it grows to a length of about three feet. Its four long ray-like limbs seem to be quite useless to it, and it swims by means of its tail, along the upper part of which runs a narrow fin. It is a creature of prey, feeding upon other fishes, and when food is plentiful, it just takes one bite out of the lower part of their bodies and no more.