172 Mr. L. E. Griffin on the 



any body a contraction of the radial muscles within each 

 segment would pull the central portion away from the opposed 

 surface and cause a vacuum to be formed between the segment 

 and the surface. The adhesive power of any one segment 

 must be slight ; but there are from sixty to one hundred 

 segments in each tentacle, half of which would probably be 

 in a position to hold ; and there are thirty-eight tentacles in 

 the group. Thus the combined adhesive power of all the 

 segments is very great. 



It seems to me probable that we see here the beginnings of 

 the suckers of the Dibranchiates. As these Cephalopods 

 became more active and predatory the simple sucker would 

 naturally have been modified to form an organ better adapted 

 to quick and sure seizing of the prey. Yet the principle of 

 action is the same in the Nautilus and the Dibranchiates. It 

 seems probable that the projecting portion of each segment of 

 the tentacle formed a single sucker. Growth would cause 

 these to take alternating positions. Possibly from each 

 segment several suckers were formed by subdivision of the 

 adhesive surface and the development of each portion into a 

 sucker. These may have remained arranged in transverse 

 rows, as in Sepia and other forms. 



This theory of the formation of the suckers of the Dibran- 

 chiates necessitates our regarding the arms of the Dibran- 

 chiates as each corresponding to a single tentacle of the 

 Nautilus, and not to groups of tentacles. The structure, 

 arrangement, and relations of the arms support the first view 

 far more than the second. Comparison with the processes of 

 change in other orders weakens the latter theory, while 

 strengthening the theory that a few tentacles gradually in- 

 creased in size while the remainder were crowded aside and 

 reduced. 



The ocular tentacles present several differences of structure 

 from the digital tentacles. Their sensitiveness is much 

 greater. The greater depth of the annular grooves on the 

 inner side is immediately noticed. Willey has found that 

 the sides of the groove are ciliated. In some instances I find 

 the cilia extending over the surface between the grooves. 



Of internal structure two points are especially remarkable. 

 One is struck by the ease with which the tips of the ocular 

 tentacles break off. This seems to be the result of a peculiar 

 arrangement. In the plane of the annular groove the ad- 

 joining segments are separated by what in sections appears 

 as a plain line along which the tissues are evidently weaker 

 than elsewhere. In apparently normal cases there is no 

 separation of the tissues, but the weak line appears sharply 



