The Book of Household Management by Mrs. Beeton

202. THE BODIES OF FISHES are mostly covered with a kind of horny

scales; but some are almost entirely without them, or have them so minute as to be almost invisible; as is the case with the eel. The object of these is to preserve them from injury by the pressure of the water, or the sudden contact with pebbles, rocks, or sea-weeds. Others, again, are enveloped in a fatty, oleaginous substance, also intended as a defence against the friction of the water; and those in which the scales are small, are supplied with a larger quantity of slimy matter.