58 



LECTURE IV, 



ON THE CLASSIFICATION OF ANIMALS. 



THE VEETEBEATA; OE PISCES, AMPHIBIA, EEPTILIA, AVES, AND 



MAMMALIA. 



In the rapid survey of the animal kingdom with which we have 

 been occupied in the preceding lectures, I have, for reasons 

 which will be obvious by and by, taken group by group, and 

 considered each separately upon its own merits, without attempt- 

 ing to say anything of the characteristics of the larger divisions 

 into which these classes may be arranged. That is a point to 

 which I shall return on a future occasion. 



But with those animals which are called " vertebrated," 

 such a course as this would involve a great and unprofitable ex- 

 penditure of time and much repetition ; because the five groups 

 of animals which pass under this name — the classes Pisces, Am- 

 phibia, Reptilia, Aves, and Mammalia — are obviously united and 

 bound together by many common characteristics, and are well 

 known to be so connected. I shall commence the present 

 lecture, therefore, 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 

 the.-' animals endowed with a sufficiently high organization 



