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



referred, viz. the structure and signification of the sheathed ce- 

 phahc tentacula. 



M. Valenciennes*, confirming without further detail my de- 

 scription of these organs, proposes a diifcrent explanation of their 

 analogies : he compares the sheath of the tentacle to the aceta- 

 bulum or sucker on the arms of the Dibranchiate Cephalopods, 

 and the tentacle itself to the caruncle of the sucker, and he di- 

 vides these supposed modified and gigantic acetabula into eight 

 groups, corresponding with the eight arms of the Octojms. I have, 

 in reference to this ingenious view, re-examined more minutely 

 the intimate structure of the tentacula of the Nautilus, and com- 

 pared it with that of the arms of the higher Cephalopods, and 

 the following account, from the Hmitcrian Lectures for 1843, of 

 the structure and homology of the tentacula of the Pearly Nau- 

 tilus may be acceptable to your readers whose attention has been 

 recently called to the anatomy of that rare and interesting mol- 

 lusk by the letter of Prof. Vrolik. 



" The anterior or muscular division of the Nautilus, which may 

 be termed the head, forms a strong and wide sheath, containing 

 the mouth and its more immediate appendages ; its inner surface 

 is for the most part smooth, the outer one divided and extended 

 into many parts or processes. The chief of these forms a broad 

 triangular muscular plate or hood covering the upper part of the 

 head, and presenting a middle and two lateral superficies ; the 

 former being traversed by a median longitudinal furrow, indica- 

 ting the place of confluence of the two large hollow tentaculiferous 

 processes of which it is composed. Each side of the head sup- 

 ports a group of perforated processes or digitations, the largest 

 of which is next the hood, and the rest decrease in size as they 

 descend in position. Exclusive of the short, subocular, perforated 

 processf, the digitations are eighteen in number on each side, 

 disposed irregularly, but all directed forwards, some not reaching 

 as far as the anterior margin of the head, others projecting a few 

 lines beyond it. They are of a conical, subtrihedral form, and are 

 hollow : the large one next the confluent pair which forms the hood 

 has, like that part, a papillose outer sui-face. Each process con- 

 tains a long and finely annulated tentacle of a subtrihedral form, 

 with the inner surface incised, as it were, by deeper and fewer cuts 

 (fig. 1. c), so as to present the appearance of a number of close- 

 set transverse plates, slightly indented by a median longitudinal 

 impression (fig. 2./). This modification must increase the pre- 

 hensile and sentient properties of the inner surface of the tentacle, 



* Nouvelks llecherches sur le Nautile Flambe, Archives du Museum, 

 4to, 1839. 



f Particularly described and shown not to be tentaculiferous by M.Va- 

 lenciennes. 



