Mr. J. D. Macdonald on the Anatomy o/Nautilus. 397 



this respect they correspond with the Echinidan and Asteridan fami- 

 lies. He shows that in the Aphroditacese the general cavity is never, 

 under any circumstances, used as an incubatory chamber. In tliis 

 point of their generative history the Echinidse and Asteriadse exactly 

 agree with the Aphroditacese. 



The author regrets, that, in consequence of the difficulty of obtain- 

 ing specimens, he is obliged to defer to a second memoir many special 

 points of anatomical structure and ph^'siological relations, the deter- 

 mination of which he still feels to be necessary to the complete 

 history of the segmental organ in the Annulose and Radiated classes. 



February 19, 1857.— Dr. \V. A. Miller, V. P., in the Chair. 



"Further Observations on the Anatomy and Physiology o^ Nau- 

 tilus" By John D. Macdonald, Esq., Assistant Surgeon R.N, 



Both Professors Owen and Valenciennes noticed that the hollow 

 subocular process of their specimens of Nautilus Poinpilius was not 

 tentaculiferous, and I may be permitted to say that this was also true 

 of several examples of Nautilus Pompilius, and one of N. macrom- 

 phalus, examined by me. But there is still another matter worthy 

 of remark with reference to this process, namely, that its cavity may 

 be traced downwards, inwards, and a little forwards, to within about 

 the twentieth of an inch of the auditory capsule ; indeed it would 

 appear as though provision had been made for the entrance of sono- 

 rous waves through a rudimentary external ear. 



There can be little doubt that the eye itself is a modified tentacular 

 sheath, so fashioned and endowed as to become the seat of the special 

 sense of vision ; but the subserviency of such a part to the faculty 

 of hearing is much more obviously seen in the subocular processjust 

 noticed, which holds an intermediate position between the organ of 

 vision and the tentaculiferous sheaths protecting the proper organs of 

 touch. 



In a figure which accompanied this communication, the auditory 

 sac is exposed by an incision made in the groove between the funnel- 

 lobe and the base of the tentacular sheaths. The subocular process 

 is slit open to the bottom of its cavity, so as to show its termination 

 in close proximity to the ear-sac. The interior of the tube is lined 

 with a glandular meml)rane tlirown into small folds, disposed longi- 

 tudinally, but the exterior of the process is quite smooth like the rest 

 of the integument. 



I have often had some little difficulty in detecting the otolithes or 

 otoconia, as the case may have been, in gasteropods long immersed 

 in spirits or otiier preservative fluids ; but in a specimen of N. Pom- 

 pilius, kef)t for many months in strong gin, although the soft parts 

 were far from being well preserved, I was enabled at the first attempt 

 to remove the contents of the auditory sacs, and the minute elliptical 

 otoconial particles, identical in character with those of N. macrom- 

 phnlus, were very distinctly seen under the microscope. 



In a former paper, I first noticed my discovery of simple auditory 

 capsules in, as I then supposed, the N. vmhilicatus ; but I find that 



