FISHES. 333 



CHAPTER XIV. 



DIVISION I. -VERTEBRATES. 



CLASS IV. — FISHES. 



. Being destined to pass their lives in the water only, fishes 

 ATe provided with a structure and organs adapted to the element 

 in which they reside. The heart has hut one auricle and one 

 ventricle, and the blood passes from the body into the first, 

 whence, by means of the ventricle, it is conveyed to the gills, 

 which perform the functions of lungs. Situated upon each side 

 of the posterior part of the head, the gills consist of semicircular 

 arches of bony or cartilaginous substance, to which are attached 

 membranes, divided into little fibrils or fringes, to which the blood, 

 after it comes from the heart, is distributed in very minute vessels. 

 By the action of the mouth a constant current of water is passed 

 over the gills, and the air contained in this water exerts an influ- 

 ence on the blood circulating through them, producing changes 

 similar to those effected in the lungs of other animals by inhaled 

 air. The blood does not return to the heart from the gills, but, 

 being collected into one large artery, is passed down along the 

 spine, to be distributed to the different parts of the body, and 

 again returned to the heart through the veins. 



The covering of fishes is a strong, thick skin, in addition to 

 which most of them have scales, arranged one over another like 

 shingles on the roof of a house. A thin coating of slime or mucus 

 spread over their bodies protects them from immediate contact 

 with the water. Fishes have the senses of sight, hearing, smell, 

 taste, and touch ; the latter very imperfectly, as they possess no 

 organ which seems adapted for its exercise, except the snout and 

 mouth, and a sort of feelers, which some species have growing 

 around the mouth. The skeleton is constructed of bones, com- 



