36 INTRODUCTION. 



tre; and this is the case even when they consist of but two series, 

 for then the two faces are similar. They approximate to the homo- 

 geneity of plants, having no very distinct nervous system or particu- 

 lar organs of sense; in some of them, it is even difficult to discover 

 a vestige of circulation; their respiratory organs are almost univer- 

 sally seated on the surface of the body, and the lowest of the series 

 are nothing but a sort of homogeneous pulp, endowed with motion 

 and sensibility. 



