52 



merous and singularly disposed tentacles of Nautilus, we have also examples of a 

 recurrence of structures heretofore unknown among the Cephalopods, and whose 

 analogues are to be sought for in inferior groups. And here again, as in the case 

 of the eye, after searching in vain among the Pteropodous genera*, we are com- 

 pelled to admit the claims of the Gasteropods to a closer alliance with the highly 

 organized class whose affinities the Pearly Nautilus has tended so materially to 

 elucidate. Thus Doris, Thethys, and Tritonia, each present examples of sheathed 

 and retractile tentacula ; and in the former of these genera they have the same 

 peculiar structure as is displayed in the ophthalmic tentacles of Nautilus. See 

 Cuvier, Mem. sur le Doris, p. 12. pi. 2. fig. 1. 



On a consideration of the generative system, it will appear that, as far as re- 

 gards the female, the Pearly Nautilus does not recede materially from the Ce- 

 phalopodic type ; and the differences which it exhibits in this respect, having 

 already been noticed in the description of the organs, need only to be alluded to 

 here : but on this subject it may be remarked, that the Pectinibranchiata in 

 their dioecious mode of generation approximate closer to Nautilus than the other 

 Gasteropoda, and present a similar laminated glandular organ in the branchial 

 cavity, whose office is supposed to be to secrete the receptacles of the ova after 

 they are expelled. As it is in these receptacles that the rudimentary shell is 

 developed in the Pectinibranchiata, the similarity in texture and bulk of its 

 testaceous appendage may render, in Nautilus, a similar nidus necessary for the 

 protection of its ova, until that process be completed. 



In whatever degree the shell is developed in the Cephalopodous Mollusks, we 

 find it invariably characterized by the symmetry so peculiar to the disposition 

 and general form of their soft parts : but the extent to which the Pearly Nau- 

 tilus is covered by its shell, and its close attachment to it, indicated the atfinity 

 to the Gasteropods in too strong a manner to escape the penetration of Aristotle, 



* The retractile tentacles of Clio nre constructed on a plan very different from those of Nautilus. 

 The considerations, indeed, on wliich the Pteropoda have been placed in the ' Regne Animal' next in 

 order after the Cephalopoda, and preceding the Gasteropoda, appear to be slighter than liave usually 

 influenced the immortal author of that work in the position of his groups. Their swimming hke the 

 former animals is a relation of analogy ; whilst their inferiority to some, at least, of the Gasteropodous 

 families is evidenced by the doubtful nature of their organ of vision, and by their hermaphrotlitical 

 mode of generation. 



