Prof. Owen on the Structure of the Pearly Nautilus. 311 



processes of the oral sheath the cavities of those appendages of 

 the arms of the Dibranchiata. According to the French mala- 

 cologist, the anterior circumference of the head or oral sheath in 

 the Nautilus represents fom- of the eight arms developed there- 

 from in the Dibranchiata ; and the two dorsal arms consist each of 

 two enormous acetabula, whose cavities are deepened into tubes, 

 and whose caruncles are produced into tentacula as highly orga- 

 nized in regard to their nerves and muscles, as are the acetabu- 

 liferous arms themselves in the higher order of Cephalopods. The 

 four other arms of the Octopus are represented, according to ]M. Va- 

 lenciennes, by the four groups of tentacula which are included 

 within the oral sheath in the Nautilus. Such is not, however, the 

 place of origin of any of the eight arms in the Dibranchiata; nor 

 is it conformable with the general law^ of development, that a pre- 

 hensile organ, consisting of two large and highly complicated 

 acetabula in a low organized Cephalopod, should support two 

 hundi-ed smaller and more simple suckers in the higher organized 

 species," 



Such are the additional facts and arguments which lead me to 

 reject the new views propounded by j\I. Valenciennes respecting 

 the sheathed tentacula of the Nautilus, and to uphold the inter- 

 pretation which I originally gave of their essential nature and 

 analogies. In dissenting from my esteemed friend, I must ac- 

 knowledge that his pecuHar views chiefly stimulated me to that 

 closer investigation of the structure of the tentacula which has 

 led to my observation of the special modification of their internal 

 surface and of the arrangement of their muscular structure, which 

 had been overlooked both by myself in the tu'st description of the 

 Nautilus, and by M.Valenciennes. I shall, in the second edition 

 of my memoir on its organization, add some other particulars of 

 the minute anatomy of the Nautilus, and meanwhile look foi'\\'ard 

 ^dth great interest to the extended memoir of Professors Vrolik 

 and Van Breda, especially in reference to the homologies of the 

 sheathed tentacula. I should wish, in conclusion, to call their 

 attention to the ' elongated pyriform sac ' described by me at 

 p. 35. pi. 5. fig. 18. connected with the ventricle, and which does 

 not appear to have attracted the attention of M. Valenciennes. 

 I have the honour to be, Gentlemen, 



Your veiy obedient servant, 



Richard Owen. 

 London, Sept. 13, 1813. 



