Fig. 232.—The Common Eel
Herrings are captured principally off the north and east coasts, but the pilchards, which are often confused with them, and even at times sold under the same name, are caught chiefly off the coast of Cornwall.
Although the Eels (Anguillidæ) are so readily distinguished by their general form and appearance, yet it may be advisable to call attention to one or two of the leading characters that would possibly be overlooked by an ordinary observer, and in doing this we ask the reader to note that our remarks apply to the true eels only, and not to the sand eels and other fish that may be confused with them.
The elongated bodies of the Anguillidæ are covered with a slimy skin that is apparently scaleless, but an examination with the microscope will show that there are small scales embedded in it. The dorsal and ventral fins extend to the tail, and the pelvics are absent; the gill-slits, which are very narrow, are at the base of the pectorals.
It might well be expected that eels would be possessed of some form of accessory breathing apparatus, seeing that they can live so long out of water, but this is not the case. They have, however, a pouch-like gill-cavity which can be inflated and filled with water by the fish, thus keeping the gills moist and functional. In most other fishes the gill-chamber is not capable of holding water, and thus the gills soon become dry and sticky, so that they adhere together and fail to absorb the necessary oxygen when the fish is out of water.
Thus the Eel (Anguilla vulgaris), in the remarkable migrations for which it is noted, is capable of travelling over dry land for considerable distances in search of suitable homes.
If an eel be taken out of the water, these gill-pouches will be seen to swell out almost immediately, and remain filled with water as long as the fish is kept on land; but when it is returned to its natural element, it will at once discharge the water that kept its gills moist, and which has become foul with the products of respiration, and, with a few vigorous gulps, renew the supply.
Eels spend their breeding season, which extends from July to September, in salt or brackish waters; and early in the following summer, the young, which are now called elvers, and measure from three to five inches in length, ascend the rivers, travelling enormous distances and overcoming obstacles that we might well expect to be insurmountable. Thus they perform two migrations annually, though it is thought by some observers that the adult never returns to the sea, but dies soon after it has deposited its spawn.