CATEGORIES OF CLASSIFICATION. 35 



constrictions, forming movable rings ; but the 

 striking features of the animal are always above 

 or below, and especially developed on the back. 

 Any collection of Insects or Crustacea is an 

 evidence of this ; being always instinctively ar- 

 ranged in such a manner as to show the pre- 

 dominant features, they uniformly exhibit the 

 back of the animal. The profile view of an 

 Articulate has no significance ; whereas in a 

 Mollusk, on the contrary, the profile view is 

 the most illustrative of the structural char- 

 acter. 



In the highest division, the Vertebrates, so 

 characteristically called by Baer the Doubly 

 Symmetrical type, a solid column runs through 

 the body with an arch above and an arch below, 

 thus forming a double internal cavity. In this 

 type, the head is the prominent feature ; it is, 

 as it were, the loaded end of the longitudinal 

 axis, so charged with vitality as to form an in- 

 telligent brain, and rising in man to such pre- 

 dominance as to command and control the whole 

 organism. The structure is arranged above and 

 below this axis, the upper cavity containing, as 

 we have seen above, all the sensitive organs, 

 and the lower cavity containing all those by 

 which life is maintained. 



While Cuvier and his followers traced these 

 four distinct plans, as shown in the adult ani- 



