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.
In summer the rivers in which it lives often dry up altogether, and the mud at the bottom is baked as hard as a brick by the rays of the sun. So, as soon as the water begins to get shallow, the animal burrows deep down into the mud, curls itself up like a fried whiting, and falls fast asleep for several months, just as hedgehogs and dormice do during the winter in cold countries. Then, when the rainy season comes and the rivers fill up again, it comes out from its retreat and swims about as before. It is from this habit that it gets its name of mud-fish.