This name signifies ‘round-mouthed,’ while the Lampreys themselves form the still smaller division _Marsipobranchii_, which means ‘pouch-gilled,’ these two being among the most evident characters of the creatures concerned. They have no true jaws, the circular mouth being supported by a ring of cartilage, and provided with a rasp-like tongue that enables them to divide their food. They have no true bone in their bodies, the simple skeleton, without limbs and ribs, being entirely cartilaginous, and the rudimentary skull is not movable on the dorsal cartilage. Their bodies are elongated and eel-like, with a single medial fin, supported by fine cartilaginous rays, and with seven little slits on each side of the neck, communicating with as many gills in the form of little pouches. The mouth is suctorial, presenting, when open, a circular adhesive disc, by which the animals can attach themselves to any solid object, but assumes the form of a mere slit when closed. The young differ from the adult in a few points of structure. Thus they have no eyes, and the long fin, divided in the adult, is continuous. With the above characteristics in mind, there will be no danger of confusing the lampreys with the eels and other similar fishes. There are three or four British lampreys, two or three of which inhabit fresh water. Their habits do not seem to be well understood, but it appears certain that the Sea Lamprey (_Petromyzum marinus_), which reaches a length of from one to two feet, ascends rivers to spawn, while the smaller River Lamprey (_P. fluviatilis_) has been caught in the sea; and it is probable that the migrations of both, together with the sojourn of the young of the former for a longer or shorter period in fresh waters, have been the cause of the widespread confusion between species. Lampreys are carnivorous creatures, and attach themselves to fishes by their suctorial mouths, and rasp away the flesh. They have also been known to attack bathers. [Illustration: FIG. 226.--THE SEA LAMPREY] Passing now to the true fishes, we must first study the general features of the group by which they are to be distinguished from other animals. Since there are so many creatures outside this class that are more or less fishlike in some respects, it becomes no easy matter to give a concise definition of a fish, and the shortest satisfactory description must necessarily include several points of structure. Thus, we may define a fish as a cold-blooded vertebrate that does not undergo metamorphoses, with limbs modified into fins, possessing also median fins on the dorsal and ventral surfaces, having distinct jaws, a heart with two chambers, and breathing by gills. To this we may add that the young are generally produced from eggs, and that the skin is covered with scales or bony plates, or is naked.