GASTROCHiENA. 131 



respect to the allied genus Clavagella, in which the valves 

 of the shell adhere to the tube, that we caunot otherwise 

 understand their growth, since it becomes necessary that 

 the wall of the tube should disapj^ear ; then it is replaced 

 by the shell. He describes a peculiar organ connected with 

 the mantle, which he believes to secrete an acid for the 

 purpose of destroying a part of the tube, and permitting of 

 the enlargement of the cavity inhabited by the animal. 

 This organ he supposes to have alternations of activity and 

 repose. Such an explanation, however^ is too hypothetical 

 to be admitted without question, and the purpose of the 

 structure, which the distinguished French malacologist 

 terms an acid-secreting organ, is by no means clearly made 

 out. We should rather be inclined to believe that the en- 

 largement of the tube was effected by some process of 

 absorption and replacement ; but for light upon this as 

 upon many other obscure though highly interesting pheno- 

 mena in the economy of these mollusks, we must wait for 

 further researches and careful observations on the habits of 

 living animals. In the British seas GastrocJieena: are not so 

 generally distributed or so plentiful as to afford convenient 

 opportunities for such observations ; but the naturalists 

 who live on the coasts of the Mediterranean would do well 

 to direct their attention to the subject. 



The genus, though including comparatively few species, 

 is almost cosmopolitan, having representatives in most seas. 

 Geological researches shew that it was even less extensively 

 developed in the pre-adamic epochs than now. 



