43 



tures of its shell with a material having the same chemical compo- 

 sition as the original shell, and diifering in mechanical properties 

 only in being a little more opake. 



7th. The repairing material is laid on from without the shell, as 

 it should be according to the theory of the function of the mem- 

 branous arms as calcifying organs, 



8th. When the embryo of the Argonaut has reached an advanced 

 stage of development in ovo, neither the membranous arms nor shell 

 are developed. 



9th. The shell of the Argonaut does not present any distinctly 

 defined nucleus. 



Mr. Owen finally proceeded to consider the validity of the best 

 and latest arguments advanced in favour of the parasitism of the 

 Cephalopod of the Argonaut, and commenced with those published 

 in the Proceedings of the Zool. Society for 1836, p. 122. 



" Mr. Gray states, 1st. ' The animal has none of those peculiarities 

 of organization for the deposition, formation, and growth of the 

 shell, nor even the muscles for attaching it to the shell, which are 

 found in all other shell-bearing Mollusks ; instead of M'hich, it 

 agrees in form, colour, and structure with the naked Mollusks, 

 especially the naked Cephalopods.' 



" To this statement it need only be replied, that the Cephalopod of 

 the Argonaut possesses two membranous expansions, having the 

 same structure as the calcifying processes of the mantle in the tes- 

 taceous Mollusks, and which Madame Power and M. Sander Rang 

 compare to the lobes of the mantle of Cypraa ; and that the Cepha- 

 lopod in question, instead of agreeing in structure with the naked 

 Cephalopods, differs from them precisely in the presence of conspi- 

 cuous and largely-developed organs, which present the closest cor- 

 respondence in form and structure with the calcifying membranes 

 of the Cowries and other testaceous Mollusks. 



" 2ndly. Mr. Gray asserts, ' that the shell of the Argonaut is evi- 

 dently not moulded on the body of the animal usually found in it, 

 as other shells are.' 



" This assertion, like the preceding, is directly opposed to the fact. 

 But at the time when it was recorded in our Proceedings, Mr. Gray 

 had probably not examined the young Argonaut. . Yet the analogy 

 of other testacea might have indicated to him that it was essential to 

 see the young MoUusk before the degree of correspondence between 

 the animal and its shell could be definitively pronounced upon. 

 Most shell-bearing Gastropods, like tlie Nautilus and Argonaut, 

 withdraw their bodies in the progress of growth from the contracted 

 apex by which their shell commenced, and differ accordingly in form 

 from that of the original cavity of their shell. The mode in which 

 the vacated part of the shell is dealt with in different Mollusks is ex- 

 tremely various, and reducible to no common law ; in the genus 

 Magilus, e. g. it is solidified : in some species of Helix, Bidinus, and 

 Cerithitim, the deserted part of the shell, after being partitioned off, 

 is decollated : in the Nautilus, &c., it iscamerated. Was it at all im- 

 probable that in the Argonaut some other condition of the vacated 



