230 DENTALIAD. 
statement of position is of importance in coming to a con- 
clusion as to the mode of entry of the water. But if the 
position of the organs of Dentalium be examined under the 
view of the water approaching the branchiz under the mantle, 
as in the ordinary Gasteropoda, they will be found to be the 
reverse of what I have stated to be the usual natural position ; 
the heart will be found at the anterior end of the branchial 
cavity instead of at the posterior, and nearest to the entrance 
of the water instead of furthest from it: here is a subversion 
of the order of nature im respect to the position of these essen- 
tial organs: how are they to be placed in harmony with her 
laws? The solution of this question is simple: we have only 
to consider that the water in this genus flows to the branchize 
by the posterior aperture instead of at the front; this view 
removes every difficulty, and may be regarded as a demon- 
stration of the fact, which is satisfactory and decisive, because 
it is founded on the organization which nature has conferred 
on these animals. 
I will state some facts in support of the conclusion that the 
branchize in Dentalium receive the water posteriorly. I admit 
that, notwithstanding a constriction, it may possibly enter in 
front under the mantle and be discharged posteriorly, and vice 
versd; but this action would be contrary to the natural posi- 
tion of the organs and to the evidence I shall now adduce. But 
first it will be necessary to mention the mode of fixture of the 
animal to the shell: this is not at the centre, as in the spiral 
Gasteropoda, but at the posterior end, a little more than an 
eighth of an inch from the terminus, where, on the inner 
surface, may be seen the striz, in the hollows of which the fine 
filaments, issumg through the mantle and proceeding from the 
longitudinal elastic riband running from the foot, are depo- 
sited ; and, together with the strong sphincter of the posterior 
process, which is imbedded in an indentation not visible from 
without, firmly secure, by constriction, that end of the animal 
to the shell. This is a strikmg example of the admirable 
adaptations of nature of the organs of animals to their wants 
and ceconomy ; for if this animal was fixed to the middle of 
the shell as in the spiral ones, the contractibility of the poste- 
