CLASS V. PISCES, 



419 



We have now eome to tlie fifth and last Class of the Yertebrata 

 We have seen how this form of structure has been variously 

 adapted to many kinds of animals that live upon the land, and those which spend a great part 

 of their time in the 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 branchiae and lungs. But we now approach a 

 class of innumerable species, to which the oceans, seas, lakes, and rivers are a perpetual home — 

 those wliich 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 water 

 instead of air. 



A Fish may be defined as a Vertebrate animal breathing through the medium of Avater, by 

 means of branchia?, 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 oft'ers itself to our attention is the breathing apparatus, called the branchice. They arc 

 situated on each side of the neck, and consist of numerous laminaj fixed on arches. These 

 lamina} 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 



