A very numerous and various class consisting of small animals, breathing through lateral spiracles, armed on all sides with a bony skin, or covered with hair; furnished with many feet, and moveable antennae (or horns), which project from the head, and are the probable instruments of sensation.

Animals of slow motion, soft substance, able to increase their bulk and restore parts which have been destroyed, extremely tenacious of life, and the inhabitants of moist places. Many of them are without a distinct head, and most of them without feet. They are principally distinguished by their tentacles (or feelers). By the Ancients they were not improperly called imperfect animals, as being destitute of ears, nose, head, eyes and legs; and are therefore totally distinct from Insects.

Animals that suckle their young by means of lactiferous teats. In external and internal structure they resemble man: most of them are quadrupeds; and with man, their natural enemy, inhabit the surface of the Earth. The largest, though fewest in number, inhabit the ocean.

Always inhabiting the waters; fish are swift in their motion and voracious in their appetites. They breathe by means of gills, which are generally united by a bony arch; swim by means of radiate fins, and are mostly covered over with cartilaginous scales. Besides the parts they have in common with other animals, they are furnished with a nictitant membrane, and most of them with a swim-bladder, by the contraction or dilatation of which, they can raise or sink themselves in their element at pleasure.