CATEGORIES OF CLASSIFICATION. 385 
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 Pla as shown in the adult ani- 
