DIGITAL AND OPHTHALMIC TENTACLES. 773 



the two nerves are for a very short distance enclosed in a common perineurium. The 

 next nerve dorsad of in and iv is II, which has a length of about 21 mm. from its origin 

 to its insertion into the base of the second tentacle. Then comes I with a length of 

 28 mm. giving off collateral branches to the substance of the hood. By cutting through 

 and turning back the roots v, vi, vn and vm, those of ix, x, xi and xii are exposed 

 in sequence at a deeper level, IX occurring between VIII and VII, x between VII and 

 vi, XI in front of vi ; xn lies at a slightly deeper level, and bears the same relation 

 to xi that IV does to V. The sequence of IX, X and XI is as striking as that of 

 v, vi, vii and vm but I am inclined to lay special stress upon the latter sequence. 

 The root of xvn lies opposite to VIII but at a deeper level than the series IX, X and XI. 

 Without attempting the impossible task of proving to complete satisfaction the 

 truth of the entire sequence suggested by me, I think it is legitimate to select a small 

 group as a test case, and in this sense I may repeat the fact, which is not at all 

 obvious without actual demonstration, that the nerves v, vi, VII and VIII occur in 

 indubitable topographic sequence (PI. LXXXIL fig. 6). 



Before devoting a few lines to a consideration of the inner whorl of the cephalo- 

 podium, it is necessary to turn our attention to the ophthalmic tentacles which occur in 

 front of and behind the eye. These resemble the digital tentacles in shape and annulation, 

 but differ, in that the sulci are much deeper on the side corresponding to that on which 

 the suctorial ridges of the former are placed, so that the tentacles are converted into 

 distinctly lamellate appendages, and when examined in the fresh state under the microscope, 

 the grooves and lamellae are found to be covered with vibratile cilia (PI. LXXIX. fig. 14). 

 The suctorial ridges of the outermost digital tentacles may in favourable specimens 

 be seen to become more and more prominent and lamelliform towards the free ends, 

 and I have observed that when the main portion of the tentacles has been affixed 

 to a surface, the tips of the tentacles which probably exercise some kind of tactile 

 function were free. I have never seen cilia at any part of the true digital tentacles. 



The ciliated lamellae of both the pre-ocular and the post-ocular tentacles are directed 

 forwards, and the shafts of the tentacles when extruded are kept in an attitude of 

 attention, the anterior being directed obliquely outwards and forwards, the posterior 

 obliquely outwards and backwards, but nearly or quite straight (cf. PI. LXXVII. figg. 3, 6). 



I came to the conclusion 1 that the ophthalmic tentacles of Nautilus while homologous 

 with the adhesive digital tentacles have become modified to serve as accessory olfactory 

 organs, and that the ciliated ridges represent olfactory lamellae. In the living state the 

 ophthalmic tentacles afford a strong contrast to the brown digital tentacles by reason of 

 their nearly uniform white colour; there is a small trace of brown pigment in the 

 annulations and at the edges of the lamellae, but when viewed under water the general 

 colour is white. They are also characterised by increased sensibility. Whereas the digital 

 tentacles can be touched without being retracted in consequence, the slightest contact 

 with a foreign body occasions the instant withdrawal of the ophthalmic tentacles into 

 their sheaths. 



1 Willey, A., "The pre-ocular and post-ocular tentacles.. of Nautilus." Quart. J. Micr. Sc. Vol. 40, 



1897, p. 199. 



101—2 



