ON MOLLUSCA. 143 
insects, but in a different direction. Nevertheless, we think 
that if, in truth, the structure of the first genera possesses 
something of that of vertebrated animals, in the rudiments of 
skeleton which envelope the brain, yet the last are conside- 
rably degraded, and pass quickly into the lowest type of 
animals, so that we may place them in a line parallel to the 
animals articulated externally, and as it were passing to the 
actinozoaires, by the ascidia, &c. 
Although nearly to these latter times, as we shall see in 
noticing the history of this part of zoology, these animals have 
been very much neglected, for the almost exclusive examina- 
tion and study of their envelopes or shells, they do not the 
less present a very considerable degree of interest in many 
respects, so as to induce us to smooth the difficulties involved 
in their investigation. ‘Thus the anatomist, and more espe- 
cially the physiologist, will find that their study throws no in- 
considerable light upon certain general questions of the first 
interest and importance : they will find, for instance, the organ 
of hearing reduced, so to speak, to its most simple expression, 
to that which is absolutely essential to it,in the sepza and the 
neighbouring genera ;—they will find the principal organ by 
which the blood is moved divided into several parts, and 
sometimes presenting the singular disposition of appearing to 
be traversed by the intestinal canal. In studying the organs 
of generation in these animals, they may form more correct 
notions of hermaphrodism, complete or incomplete ; thus, in a 
great number the organs, both male and female, exist in the 
same individual, though such individual cannot fecundate it- 
self; so that the species is necessarily composed of two indi- 
vidual hermaphrodites, while in others, in which the female 
organs alone are perceptible, generation takes place by means 
of a single individual. 
Geology will derive not less advantage from the minute 
study of the shells or envelopes of the mollusca, to assist in 
15 
