230 DENTALIADJE. 



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 branchise 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 in 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 branchise 

 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 

 branchise 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 

 versa 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 strise, in the hollows of which the fine 

 filaments, issuing 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 striking example of the admirable 

 adaptations of nature of the organs of animals to their wants 

 and (economy ; for if this animal was fixed to the middle of 

 the shell as in the spiral ones, the contractibility of the poste- 



