The round, soft mouth is suctorial, the tongue acting as a piston. By suction it can anchor itself to a rock, and allow the long body to float freely, without being carried by the current from the place. There are seven gill openings on each side, and the whole breathing apparatus so arranged that the animal can live some days out of the water. In some parts of England it is in demand for the table, and “lamprey pie”[31] is esteemed a great luxury. The American lamprey is similar, and the flesh good, but in less demand in our markets. During the breeding season those about the estuaries go up stream as do the shad, and by rolling stones together construct large conical nests[32] for the protection of their young. With apparently little adaptation for such architecture they accomplish wonders in that line.
THE LAMPREY.
Class I.—Pisces. The first clearly defined division of vertebrates is that of the fishes. They are regarded as, in some respects, the lowest of vertebrate animals. They are credited with having the least intelligence and sensibility. Their eyes, though often large, are nearly motionless in their sockets, are protected by no eye-lids, and are without the expression usual even in the animal eye. If they have ears the external parts are wanting. Sounds may reach the auditory nerve and be heard through the cranium. The other senses, as taste, touch and smell, are but slightly developed. But they are admirably adapted to the element in which they live, and the mode of life for which they were created. In no other organisms is the evidence of design in the adaptation of means to the ends contemplated, more apparent. In the number of species and variety of forms they exceed all other vertebrate animals.
The skeleton of a fish is usually divided into four parts: the head, respiratory organs, vertebral column and limbs.
The head is very suggestive, and of itself presents a profitable study. It is not hung or poised on a neck, but attached immediately to the body. In most species it is large, making a large mouth possible, but pointed, to lessen the resistance met in passing through the water. In some the eyes are quite near the nasal organ, in others farther back. In some they face laterally, in others upward. As there is no nictating[33] membrane there is neither winking nor the shedding of tears. Both jaws are, to some extent, movable and provided with osseous teeth that are usually sharp and of a spike-like form.
From the heads here presented the operculum,[34] or gill cover, is removed to show those delicate respiratory organs. The branchiæ, or gills, are situated at the sides of the head just back of the eyes, and consist of numerous and very vascular[35] plates, arranged in double fringe-like rows, fixed or attached at the base only, and so constructed, in all respects, as to expose as much surface as possible. These gills are covered with innumerable small blood vessels, to which blood is pumped from the heart, there to receive the needed supply of oxygen. The oxygen is obtained from what air circulates through the water, and not by a decomposition of the water, as some have supposed. For some species the modicum of oxygen, thus obtained, seems insufficient, and they come to the surface for more.
HEADS OF FISHES WITH GILL-COVERING REMOVED.
Notice another peculiarity. There being no neck or long gullet, the principal digestive organs are packed in the cavity near the capacious mouth, and this leaves the whole of the posterior tapering part of the body for strong muscles, that can vigorously move the caudal extremity from side to side as a propelling oar. The spine is so jointed as to allow a free horizontal and but little vertical motion.