Page:Natural History, Fishes.djvu/288

274 all round the tail. The dorsal commences at a considerable distance behind the insertion of the pectorals. The gill-opening is a small slit on each side, situated just at the base of the pectoral. The upper jaw is shorter than the lower; the mouth is furnished with small teeth in each jaw, and a few at the front part of the vomer. The whole body is covered with a thick, smooth skin, well supplied with a slimy mucous secretion. The intestine is long but straight; the air-bladder is also long, and furnished near its middle with a peculiar gland. A curious pulsating sac has been recently discovered in the tail of the Eels, with regular beats, quite independent of the action of the heart; the object of this organ is the circulation of lymph, a thin colourless fluid, having much of the nature of blood, deprived of its colouring disks. Similar organs have been observed to exist in some Reptiles, particularly Amphibia.

The Eels are properly fresh-water fishes, which migrate to the sea after they have deposited their spawn in the rivers. We have three or four species, differing a little in the form of the head, but sufficiently alike both in appearance and habits to allow of one being taken as a fair sample of the rest. The most common is the Sharp-nosed Eel (Anguilla acutirostris, .), which is abundant in most of our rivers, lakes, and marshes, as in those of continental Europe. In the waters of high mountains, however, and in countries subject to severe winter's cold, the Eel is not found, as it cannot endure a very low temperature. "During intense frosts, accompanied by a piercing east wind, thousands of