384 CATEGORIES OF CLASSIFICATION. 
them, which is the anterior and which the pos- 
terior extremity. In this type, right and left 
have the preponderance over the other diame- 
ters of the body. The sides are the prominent 
parts, —they are loaded with the most impor- 
tant organs, or with those peculiarities of the 
structure that give it character. The Oyster 
is a good instance of this, with its double valve, 
so swollen on one side, so flat on the other. 
There is an unconscious recognition of this in 
the arrangement of all collections of Mollusks ; 
for, though the collectors do not put up their 
specimens with any intention of illustrating this 
peculiarity, they instinctively give them the po- 
sition best calculated to display their distinctive 
characteristics, and to accomplish this they ne- 
cessarily place them in such a manner as to 
show the sides. . 
In Articulates there is also a longitudinal axis 
of the body and a bilateral symmetry in the 
arrangement of parts; the head and tail are 
marked, and the right and left sides are dis- 
tinct. But the prominent tendency in this type 
is the development of the dorsal and ventral 
region; here above and below prevail over right 
and left. It is the back and the lower side that 
have the preponderance over any other part of 
the structure in Articulates. ‘The body is divided 
* from end to end by a succession of transverse 
