In sharks and skates, the gills are fastened by their longer side, like the leaves of a book. In other groups there are many minor modifi cations.
In the sharks and skates there is no trace of lung or air-bladder. In the more primitive forms ancestral to the true fishes, there is a well-developed lung, which assists in the purifi cation of the blood in exactly the same way as the lung of the higher vertebrates. Most of the forms known or supposed to possess lungs (Dipneusta, Crossopterygii, Ganoideii) are now extinct. In the few living forms with this character (Neoceratodus, Lepidosiren, Protop terus, Polypterus, Lepisosteus and Amia) this lung is either of one or two lobes. It is con nected by a trachea with the pharynx. In the earliest forms this windpipe opens below the esophagus, as in the higher vertebrates, but in the other forms, the opening is transferred to the dorsal side. These early fishes were there fore literally amphibious, being capable both of breathing atmospheric air with the lungs and the air dissolved in water, with the gills. There is little doubt that from some of these forms, probably crossopterygians, the whole series of land vertebrates is descended, while from the same stock has arisen the great body of the typical or bony fishes. In the former case we have the gradual obliteration of the gills, which are developed only in embryonic or larval stages. Among the typical fishes we have the gradual degeneration of the lung and the loss of its function as a breathing-organ.
In this case we have first the reduction of the lung to a single sac, the loss of its cellular structure. It assumes in the soft-rayed fishes the form of an empty sac, sometimes divided by a cross-constriction, the anterior part con nected with the oesophagus by a very slender membranous tube. This tube is gradually lost in the spiny-rayed fishes, no trace of it usually existing except in the embryo. The air-bladder is then a hollow sac filled with air containing an excess of nitrogen, this air being secreted from the blood of the fish. The air-bladder has some value in enabling the fish to hold itself in place in the water. In pelagic fishes, and in fishes which habitually lie on the bot tom, the air-bladder is often greatly reduced, or entirely obliterated, when the fish is fully developed. The different stages of the air
bladder must be looked on as steps toward the gradual loss of the lung, an organ which be comes progressively less important in the more complete adjustment of the fish to continuous life in the water.
The Organs of Sense.— The sense-organs of the fish correspond in general to those of the higher vertebrates. The sense of taste is, however, feeble, and that of hearing is muffled and without much power of acute discrimina tion. The sense of smell resides in the nos trils, which have no relation to the work of breathing. No fish breathes through its nos trils, and only in a few of the more generalized types does the nostril pierce through the roof of the mouth. In the bony fishes the nostrils are cavities, one on either side, lined with delicate or fringed membrane, well provided with blood-vessels, as with nerves from the olfactory lobe. In most cases each nasal cavity has two external openings, either simple or with the rim of the nostril forming a papilla or barbel. The openings may he round, slit-like, pore-like, or may have various other forms. In certain families of bony fishes there is but one opening to each nostril. In the sharks, rays, and chimmras there is also but one opening on either side and the nostril is larger and highly specialized, with valvular flaps controlled by muscles which enable them *to scent actively as well as to smell passively.* In the lancelet there is a single median nos tril, a small depression at the front of the body covered by ciliated membrane. In the hagfish the single median nostril pierces the roof of the mouth and is strengthened by cartilaginous rings, like those of the windpipe. In the lam prey the single median nostril leads to a blind sac. In all other fishes there is a nasal sac on either side of the head. There is no doubt that the sense of smell in fishes is relatively acute, and that the odor of their prey attracts them to it. It is known that flesh, blood, or a decaying carcass will attract sharks and other predatory fish are drawn in a similar manner. At the same time, the strength of this function is yet to be tested by experi ments.