90 INTRODUCTION. 
ing and ability have been devoted to the 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 naturalists is adverse to the theories themselves. 
Of the animals composing the great invertebrate 
division, those known by the name of Molltjsca, 1 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 highly developed as hi either of the other 
departments. They have consequently been usually 
placed, in the order of arrangement, next after the verte- 
brate animals, and they arc 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 naturalists. 
Some of these display great learning and ingenuity, 
in the formation of the terms applied to the various 
subdivisions, and if mere words could become a substi- 
1 The word is derived from the Latin molluscus, signifying soft. 
