CHAPTER III. 



THE CHARACTERS OF THE CLASSES OF THE VERTEBRATA. 



THE ' five groups of animals which pass under the name of 

 Vertebrata the classes Pisces, Amphibia, Reptilia, Aves, and 

 Mammalia are obviously united and bound together by many 

 common characteristics, and are well known to be so connected. 

 In order to economise time and space, therefore, I shall preface 

 my account of the character of these classes by enumerating 

 the most important of those structural peculiarities which these 

 five great divisions exhibit in common. 



In the animals to which our attention has hitherto been 

 confined, the external, or integumentary and parietal, portion 

 of the blastoderm never becomes developed into more than a 

 single saccular, or tubular, investment, which incloses all the 

 viscera. So that if we make a transverse section of any one of 

 these animals endowed with a sufficiently high organization 

 to possess a nervous system and a heart, that section may be 

 represented diagrammatically as in Fig. 32 (I.) where P repre- 

 sents the parietes or wall of the body, A the alimentary canal, H 

 the heart, and N the nervous centres. It will be observed that 

 the alimentary canal is in the middle, the principal centres of 

 the nervous system upon one side of it, and the heart upon the 

 other. In none of these animals, again, would you discover, in 

 the embryonic state, any partition, formed by the original ex- 

 ternal parietes of the body, between the nervous centres and the 

 alimentary canal. 



But, in the five vertebrate classes, the parietal portion of the 



