3-4 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 



