192 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



The ix)d layer which covers the gray portion of tlie retina is composed of sleiitler rods of 

 equal size. The rods are about three times as long as the retinal cells. Each rod is composed 

 of an axial cord which stains deeply in carmine, and of a lightly staining cortex. 



The epithelium of the retina is separated from the nerve-tibre layer by a thick basement 

 membrane, through which the nerve fibres penetrate and enter the retinal cells. The nerve 

 fibres enter the broad retinal cells just above the nucleus. The buses of tlie fine retinal cells are 

 continuous with nerve fibres. 



RHINOPHORE. (Figs. 21 and 22.) 



Between the stalk of tiic eye and tlic i)rojecting posterior edge of the cephalic sheath is an 

 organ, prol)ably olfactory, which has lately been compared with the rhinophore of certain 

 Gastropoda, and to which this name is applied. It is a small pyramidal protuberance of the 

 side of the head, located on a horizontal line with the lower edge of tlie stalk of the eye. The 

 body of the eye projects over the rhinophore, completely hiding it when the Nautilus is not 

 viewed from below. The posterior side of the rhinophore is produced into a finger-like process 

 of about the same heigiit as the pyramidal liase; the whole is <S or 10 millimeters in height. The 

 process is not annulated and is "not r('tractil(>" (Willey, 1<S97, 1), and bears no resemblance 

 nor relation to the digital tentacles. It may be well to speak of this as the tentacle of the rhino- 

 phore. Just dorsal to the tentacle is a [lit •! millimeters in depth and -J millimeters in diameter 

 (the dorsal pit). On the anterior side of the base of the tentacle is the opening of another pit, 

 nari'ower ))ut much deeper than the dorsal pit. This is the fossa of the rhinophore. It is 10 or 

 1^ millimeters in depth, extending into the tissues of the hesid in a line directed inward and 

 downward, except as the last 2 or 3 millimeters of the tube turns sharply forward. The ba.'^e of 

 the fossa is near the otocyst, and Keffkkstein thinks that this is what Macdoxald mistook for 

 the otocystic canal. From just within the external opening the fossa is continued upward in the 

 center of the tentacle nearly to its tij). 



The walls of the fossa are much folded longitudinally and are lined by a single-layered 

 ciliated epithelium of slender columnar cells. Among these are man}' cells which remind one 

 strongly of the olfactory cells in some of the \ertebrata. 



The middle portion of these cells is swollen, forming a large, spherical, clear body which is 

 distingiushable in sections viewed under low powers. The proximal and distal ends of the cells 

 seem to be exceedingly slender and thread-like. Tliesc cells are limited to the walls of the fossa. 

 The epithelium of the dorsal pit is like that of the outer surface of the rhinophore. 



A large nerve appears to leave the anterior side of tlu' cerebral ganglion just under the point 

 of union of the cerebral and optic ganglia, and. lying close to the fossa, extend to the tip of the 

 tentacle of the rhinophore. 



The body of the rhinophore is comi)osed for the most })ait of dense elastic connective tissue, 

 though in the base of the organ are Sfinn' muscles. 



Keffekstein describes the tentacle of the rhino])horc as "•ein zungenformiger Lappen 

 der wie eine Klappe die Mi'indung .seines Axenkanals schliessen kann." In preserved 

 specimens the tentacle f requentlv is folded down over the mouth of the fossa, but this is apparent!}' 

 due to its being pressed upon the eye. so that the tentacle is probably in an unnatural jiosition. 



OTOCYSTS. 



The otocysts of \(iiitll iix jioinpUiKx lie upon tiie front side of the cartilage immediately 

 back of the pedal ganglia, and near the jiuiction of the latter with the cerebral and pU'urovis- 

 ceral ganglia. They are ovate in form, measuring about 3.5 millimeters i!i the direction of their 

 long diameter. The end of the auditory nerve spreads out over the surface of the otocyst. 



The otocyst is a thin walled sac sdniost completely Hlled by an immense number of elliptical 

 crystals packed closely together. The cry.stals vary in thickness between 0.0011 and 0.0066 mil- 

 limeter, and in length from 0.0033 to O.Ol-l millimeter. The crystals are composed of calcium 

 carbonate, giving characteristic chemical and light reat'tions. The\' all have the shape which 

 would be assumed b}- a perfect crystal of dog-tooth sjiar if all its angles were rounded. Very 



