CLASS Y. PISCES. 
adapted to many kinds of animals tliat live upon tlie land, and tliose wliicli spend a great part 
of their time in tlie air ; we have also noticed the curious modifications suited to animals that 
live in the water yet breathe the vital air, as well as those which are amphibious and partake 
somewhat of the two modes of respiration, by brancliiae and lungs. • But we now approach a 
class of innumerable species, to which the oceans, seas, lakes, and rivers are a perpetual home — 
those which live in the water and perish as speedily when taken from it, as the air-breathing 
animals would on being immersed in it ; those, in fact, which breathe exclusively by means of Avater 
instead of air. 
A Fish may be defined as a Vertebrate animal breathing through the medium of water, by 
means of branchiae, or gills, having one auricle and one ventricle to the heart, cold red blood, 
and extremities formed for swimming. In considering fishes, perhaps the most important thing 
which otfers itself to our attention is the breathing apparatus, called the branchice. They are 
situated on each side of the neck, and consist of numerous laminte fixed on arches. These 
laminae are covered with innumerable blood-vessels, and are so constructed as to present a con- 
siderable surface to the water, so that the blood may receive a sufficient portion of the oxygen 
contained in that element. As the water in contact with the gills becomes deteriorated, it is 
necessary that a constant current be caused to flow over them. In most fishes this is effected 
