DIGITAL AND OPHTHALMIC TENTACLES. 769 



of adhesion by means of what I have called the suctorial ridges had not then been 

 established. 



Owen's paper of 1843 was written by way of reply to Valenciennes (1841), who had 

 compared the sheathed tentacles of Nautilus not with the arms of the Dibranchs, but 

 with the suckers which are borne upon the arms, the sheath corresponding with the 

 cupule of the sucker and the tentacle with the caruncle. This interpretation was supported 

 by Professor Lankester (1883, Encyc. Brit.), and is in fact widely held 1 . For my part I am 

 unable to subscribe to the very ingenious theory of Valenciennes for reasons which I shall 

 submit later. Valenciennes not only compared the sheathed tentacles of Nautilus with 

 the acetabula of Dibranchs, but grouped them together in such a way as to show 

 a correspondence with the eight arms of an Octopod. His point of view was undoubtedly 

 ingenious though artificial. In his PI. XI. fig. 2, he represents an ideal section across 

 the cephalopodium, in which the tentacles are associated together into four paired groups 

 represented, as he claims, in actual anatomy by eight muscular peduncles. These groups 

 are as follows : — 



1. Hood and accessory sheath. 



2. Mass of the external digital lobe. 



3. Internal labial lobe. 



4. Inferior labial lobe of $ . 



Valenciennes explained his position in these words (pp. 275 — 276): — "Dans cette 

 maniere de voir je ramene au nombre ordinaire des bras des cephalopodes en general 

 le nombre de ceux de nautile, puisque les quatre-vingt-huit cirrhes, consideres avant moi 

 comme bras du nautile, ne sont plus que des appendices sortant des ventouses allongees 



en gaines Mais ne voulant pas pousser ces analogies au-dela de ce qui peut 



en quelque sorte se demontrer, je me hate d'ajouter que si les bras du nautile ressemblent 

 a ceux du poulpe par leur position autour du bee, ils sont toutefois bien differents par 

 leur forme et leur structure, malgre l'espece d'analogie que nous venons d'indiquer entre 

 les cirrhes de l'un et les ventouses de l'autre." 



Not the least noteworthy feature in regard to the digital tentacles of Nautilus, 

 especially those of the outer whorl, is their striking constancy in number and position. 

 This is the more remarkable because their number is large and their position apparently 

 arbitrary. Owing to the latter circumstance it has not been easy to arrive at a satis- 

 factory method of enumeration of the tentacles, which is a matter of statistical interest 

 and incidentally of morphological importance. 



The sheaths of the tentacles cohere in such a manner that they form a fleshy mass 

 in which the tentacles themselves appear to be arranged without regard to any law or 

 order, but a critical examination discloses, to my thinking, a possible means of inter- 

 pretation which the diagrams on Plate LXXXI. will serve to elucidate. 



The method of enumeration which I have to suggest was acquired independently 

 after many attempts to arrive at a settlement. Subsequently I became acquainted with 



1 E.g. it is the view expressed in Professor Arnold Lang's Lehrbuch der vergleichenden Anatomic, 1st edit. 

 1892, p. 691. 



