"The form of the teeth in fishes is various; in general it re presents that of an elongated cone, slightly curved inwards to assist in holding A prey which is frequently alive. Sometimes the form is that of a short and rounded tubercle, adapted for crushing ; in some fishes the teeth are so small and numerous, as to have the appearance of the hairs of a brush ; while in others they are thin and fiat like the incisor teeth in the human subject. Some fishes that are without teeth in the mouth, have them in the throat, this is particularly the case in the Carp, and the allied species of the family of the Cyprinicke generally. Fishes have cold blood ; that is to say, the blood does not, in general, rise appreciably above the temperature of the element in which they swim. It is invariably red. The heart consists of one auricle and one ventricle, which receives the blood from the veins, and sends it to the gills for renewal by the absorption of oxygen ; whence it is circulated through the body in arteries. Both the arteries and veins are perfectly closed vessels. In many fishes there is a large bladder situated within the body between the spine and the bowels ; it assumes various forms, and is always filled with air, which, in marine fishes, is principally composed of oxygen. It is supposed to be connected with the buoyancy of the animal, and hence is often called the swimming bladder ; but there are structural reasons for considering it to be the first rudimentary form of an air-breathing lung.
" The air bladder does not occur in all fishes; some fishes, and those particularly that live near the bottom of the water are without any. The swimming bladder of fishes,' says Dr. Roget in his excellent Bridgewater Treatise, ' is regarded by many of the German naturalists as having some relations with the re spiratory function, and as being the rudiment of the pulmonary cavity of land animals; the passage of connection with the oeso phagus being conceived to represent the trachea.' Hervey long ago observed that the air in birds passed into cells beyond the substance of the lungs ; thus showing a resemblance to the cellular lungs in reptiles, and the air-bladder in fishes.' M. Agassiz, in dissecting a species of Lepisosteus, a fresh water • fish of the waters of America, found the air-bladder composed of several cells, with a canal proceeding upwards into the pha rynx, and ending in an elongated slit, with everted edges, resem bling a glottis or tracheal aperture. However obvious may be these relations of structure, it is still difficult to believe that there can be any analogy in function, when it is recollected that one-fourth of the fishes known are entirely without air-bladders, and that two-thirds of the other three-fourths have neither canal nor aperture for external communication, but that all are pro vided with gills. The search for these relations of structure in animals of different classes, is among the most interesting of the investigations of the comparative anatomist."