Chap. III.] 



ICHTHYOPSIDA . 



forms, and 

 others in 

 which, a given 

 complexity of 

 structure hav- 

 ing been at- 

 tained, there is 

 a profusion in 

 the elaboration 

 of the details. 

 This truth is 

 well exempli- 

 fied in the 

 groups of the 

 Vertebrata. 



I. Ichthy- 

 opsida; these 

 are the true 

 Fishes, and the 

 Amphibia (or 

 frogs and 

 newts). In 

 them respira- 

 tion is always 

 effected by gills 

 during some or 

 the whole of 

 their life, the 

 heart never 

 has more than 

 three cham- 

 bers, and there 

 are always two 

 aortic arches at 

 least given off 

 from it. 



a. Pisces. 





