In most of the cartilaginous fishes, such as the sharks, rays, and lampreys, the gills are differently formed, the water not passing into a cavity closed by a cover, but flowing directly outwards through five (in the shark) or seven (in the lamprey) vents or spiracles. In these species also the gills are fixed, their margins being attached. Though the whole breathing apparatus of a fish is comprised in a small compass, its surface, if fully extended, would occupy a very considerable space; that of the common skate, for instance, being equal to the surface of the human body. This single fact may convince us of the numberless ramifications and convolutions of the gills, in which the water is elaborated and attenuated in the course of giving out its air; and how wonderfully Nature has contrived to effect her purpose with the greatest economy of space. [Illustration: Theoretic representation of the Circulation in Fishes.] Respiration is a species of combustion, and this must necessarily be very slow in an element which contains so small a portion of oxygen. No wonder that the circulation of the blood in fishes is equally tardy. Their heart, in comparison with _ours, is but half a one_, as it merely serves to force the venous blood into the gills--whence the aërated blood does not flow back to the heart as with us, to be rapidly and strongly propelled through the body, but proceeds immediately to the arteries. Evidently only a cold blood could be formed under such circumstances. It may seem strange that, when fishes are taken out of the water, they die from want of air; such, however, is the case. Their delicate breathing membranes collapse in the atmosphere, the blood can no longer flow as before into the innumerable small vessels with which they are interwoven, and, by rapidly drying in the air, they soon entirely lose the faculty of breathing. Thus those fishes whose gill-cover has a large aperture, die soonest in the air, while those where the opening is narrow, and more particularly those species where the gills communicate with a cellular labyrinth containing water, which serves to keep them moist, are able to live a much longer time in the atmosphere. [Illustration: The Anabas of the Dry Tanks.] [Illustration: Frog-Fish.--(Cheironectes.)] It is owing to such a moistening apparatus that _the climbing fishes_ (Anabas) live for days out of the water, and even creep up the trees at some distance from the shore, to catch the insects which serve them as food--a curious instance indeed of an animal seeking its nourishment in another element.