MANUAL OF THE MOLLUSCA. 



INTRODUCTION. 



CHAPTER I. 



ON THE POSITION OF THE MOLLUSCA IN THE 

 ANIMAL KINGDOM. 



ALL known animals are constructed upon four different types, 

 and constitute as many natural divisions or sub-kingdoms. 



1. The first of these primary groups is characterized by an 

 internal skeleton, of which the essential, or ever-present part, is 

 a backbone, composed of numerous joints, or vertebra. These 

 are the animals most familiar to us ; beasts, birds, reptiles, and 

 fishes, are four classes which agree in this one respect, and are 

 hence collectively termed vertebrate animals, or the vertebrata. 



2. Another type is exemplified in the common garden-snail, 

 the nautilus, and the oyster ; animals whose soft bodies are pro- 

 tected by an external shell, which is harder than bone, and equally 

 unlike the skeleton of fishes, and the hard covering of the crab 

 and lobster. These creatures form the subject of the present 

 history, and are called mollmca.* 



* Mollusca soft (animals), from mollis. The Greeks termed them Ma- 

 lakia, whence the modern word Malacology, or the study of shell-fish. 



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