90 INTRODUCTION. 



ing and ability have been devoted to tbe subject, the 

 analogies relied on, in all these systems, are of very 

 unequal value, being oftentimes remote and almost 

 imperceptible, and not infrequently fanciful and absurd. 

 The reasoning is fallacious, sometimes transcendental, 

 and in the main unsatisfactory, and the general opinion 

 of naturahsts is adverse to the theories themselves. 



Of the animals composing the great invertebrate 

 division, those known by the name of Mollusca,^ the 

 mollusks, or soft-bodied animals, decidedly out-rank the 

 others by the perfection of their respiratory, circulatory, 

 assimilating, and reproductive functions, while the func- 

 tions of animal life, excepting those of locomotion, 

 are as liighly developed as in either of the other 

 departments. They have consequently been usually 

 placed, in the order of arrangement, next after the verte- 

 brate animals, and they are formed according to a pecu- 

 liar type or system of organization. 



From the time of ]M. Cuvier, who was the first to 

 demonstrate the leading modifications of structure pre- 

 vailing among the Mollusks, and to found thereon a 

 truly philosophical classification, showing their several 

 distinctions and relations, many other methods have 

 been brought forward by distinguished naturahsts. 

 Some of these display great learning and mgenuity, 

 in the formation of the terms apphed to the various 

 subdivisions, and if mere words could become a substi- 



' The word is derived from the Latin moUuscus, signilying soft. 



