CLASS OF FISHES. 371 



axolotl (Pig. 869), the menobranchus, and the siren. Finally, 

 the Ceciliae, which have no limbs, and strongly resemble 

 serpents. 



Some very singular animals have been lately discovered, 

 which have branchiae and lungs like the siren, but which 

 have in place of feet only cylindrical fins, and which so re- 

 semble fishes in the whole of their organization, that most 

 zoologists have arranged them in the following class : these 

 are the lepidosirens (Fig. 146). 



CLASS OF FISHES. 



474. The fifth and last class of the primary division of 

 vertebrate animals comprises the class fishes. The circum- 

 stance of their being destined to live under water strongly 

 affects their whole organization, as is most seen in what 

 regards the apparatus of respiration and circulation ; they 

 breathe by gills, and never have lungs* at any period of their 

 lives. Their heart is composed of two cavities, the Auricle 

 and ventricle, containing only dark blood ; this blood is sent 

 to the gills, and returns from these after 'being exposed to 

 the oxygen, to be distributed to the various parts of the 

 body, no heart being interposed between the gills and the 

 other organs of the body ; their blood is cold, and their skin 

 naked, covered only with scales ; they lay eggs, that being 

 their mode of reproduction ; and finally, their limbs have the 

 form of fins. 



475. They differ considerably in the form of their 

 bodies, but the outline is generally simple : there is no neck, 

 properly so called, and the head is large ; their tail is not 

 distinguishable from the rest of the body. Some have no 

 fins, but generally we find them present, arranged in pairs 

 symmetrically at the sides or singly on the back and abdomen 

 (Fig. 381). Those in pairs represent the limbs of vertebrate 

 animals. The anterior are called pectoral fins; the inferior, 

 which vary much ir* position, are called ventral ; the asym- 

 metrical fins are the dorsal (d), and (a), and caudal (c), 



* The swimming bladder seems evidently to form a rudimentary lung. 

 E. K. 



B B 2 



