These fishes are particularly interesting on account of their gregarious habits and the enormous size of the shoals they form, a single shoal often containing millions of individuals; and they are often captured in such quantities that large numbers are sold to farmers as manure to enrich the soil. The shoals are followed closely by many larger carnivorous species that devour them in great numbers, as well as by flocks of sea birds that prey on them, and yet their numbers are not appreciably reduced by such ravages. They spawn in shallow waters near the coast, and feed principally on the crustaceans and worms of the littoral zone. Sprats were once considered to be the young of the Herring, but it is now universally acknowledged that they are a distinct species, and quite a number of characteristics have been given as a means of distinguishing between the two. The young of the herring are, however, used largely as food, for that miscellaneous mixture of fry and small species known as Whitebait consists largely of these and the young of the sprat. [Illustration: 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.