MOLLUSKS. 
CHAPTER I. 
INTRODUCTION 
OBJECT AND NATURE OE ZOOLOGICAL CLASSIFICATION* 
-CLASSES OF THE MOLLUSCOUS ANIMALS. 
NIMALS differ very much from one another 
JX not only in their form, size, and habits, 
but also in their internal structure; and we in¬ 
tuitively group them according to their resem¬ 
blances and differences, and give to each group 
a certain distinctive or characteristic name. 
Prom childhood our minds have been engaged, 
consciously or unconsciously, in the observation 
of natural objects, noting their shapes and 
qualities, and rudely comparing and classi¬ 
fying them. The differences of internal struc¬ 
ture have led naturalists to divide the animal 
kingdom into five divisions, each division being 
distinguished by some striking peculiarity of 
