CHAPTER VI. 



OF THE CLASS OF FISHES. 



515. THE fourth and last class of the Vertebrated sub-king- 

 dom, comprises the animals known as FISHES. These are destined 

 to live altogether in the water ; and this circumstance has im- 

 pressed a peculiar character on their entire organization : but 

 the most important differences which they present, when we 

 compare them with the other Yertebrata, consist in the confor- 

 mation of their apparatus for respiration and circulation. They 

 have no lungs at any period of their lives ; and they breathe by 

 gills only. Their heart contains but two cavities ; and receives 

 only venous blood. This liquid, after having been brought into 

 contact with oxygen, passes into a dorsal vessel, where no new 

 force accelerates its course through the different parts of the 

 body. Their circulation cannot, therefore, be as active as among 

 the higher animals ; and their blood is cold like that of Reptiles. 

 Their skin is covered only with scales, which, in some instances, 

 are scarcely discoverable, so that the skin appears quite bare ; 

 they have no mammary glands like the Mammalia, and they 

 are reproduced by means of eggs ; lastly, their members have 

 the form of fins. 



516. The external form of Fishes varies; but their body is 

 generally but little divided. Their head, which is as broad as 

 the trunk, is not separated from it by any narrowing like the 

 neck of the higher Yertebrata ; and their tail, by its size at its 

 origin, is not distinguished from the rest of the body. Some of 

 these animals are quite destitute of fins ; but in general we find 

 a considerable number of these organs, placed, some on the 

 central line of the back or the abdomen, and consequently single, 

 others on the sides, and arranged in pairs (Fig. 245). These 

 last represent the four limbs of the other Yertebrated animals. 



