EYE ; RHINOPHORE ; OTOCYST. 793 



In the sensory epithelium of the osphradium of Nautilus I have seen cells with large 

 vesicular nucleus and darkly staining nucleolus, bearing a great resemblance to ganglion- 

 cells, and as I have found them in special relation with the nerves it seems very probable 

 that they are in fact peripheral ganglion-cells which have retained their primitive position 

 in the epidermis. They are best seen in tangential sections (PL LXXXIII. fig. 5). 



13. Eye; Rhinophore; Otocyst. 



I do not propose to dwell in detail upon these structures but to focus attention, for 

 the special purpose of this contribution, upon their topography. They are in fact inex- 

 tricably involved in the cephalopodium, and it is of the utmost importance to appreciate 

 their character of true cephalic sense-organs, whereas the ciliated ophthalmic tentacles 

 between which the eye is placed belong to the digital series. 



The eye of the Nautilus is a hollow hemisphere with an opaque pigmented cornea 

 perforated near the centre by a small aperture which is the pupil of the eye. The size 

 of the pinhole aperture is apparently capable of slight accommodation to the intensity of 

 light as I have seen it contract from a comparatively wide orifice, 2 - 5 mm. in diameter, 

 to a mere slit with apposed margins. Passing from the aperture to the ventral border 

 of the eye is a superficial pigmented groove somewhat resembling a choroid fissure or 

 raphe. Perhaps the development of the eye would throw some light upon the meaning 

 of this corneal raphe. 



The dorsal border of the eye is convex, but the rest of the margin is produced into 

 a thin limbus. Sometimes the corneal raphe commences a short distance below the pupil, 

 and the latter then appears as an isolated subcentral orifice. 



The eye faces outwards and cannot be moved in any other direction, the optic 

 peduncle being relatively immobile and capable only of slight protraction. The whole 

 aspect of the pedunculated eye somewhat resembles a sheath of a digital tentacle, and 

 Macdonald 1 actually asserted that there can be little doubt that the eye is a modified 

 tentacular sheath so fashioned as to become the seat of the special sense of vision. There 

 is perhaps no obvious inherent impossibility in this view, especially when regarded from 

 the standpoint of those who consider the digital tentacles to be special cephalic processes, 

 but I think it is important to dismiss the idea from the mind if it should ever present 

 itself. 



The eye of Nautilus was correctly described in respect of the pinhole aperture and 

 absence of dioptric apparatus by Owen (1832), and was subsequently examined in greater 

 detail by Hensen 2 (1865). Later still it was shown by Fraisse 3 to represent a primordial 

 type of eye which is also met with among the prosobranchiate Gastropods. 



1 Macdonald, J. D., " Further observations on the anatomy and physiology of Nautilus." P. E. Soc. 

 London, vni. 1857, pp. 380 — 382. In this connection the fact may be mentioned that the optic peduncle 

 does not contain an axial nerve but separate nerves arise from the ganglion and pass through the eye-stalk to 

 the retina. 



2 Hensen, V., "Ueber das Auge einiger Cephalopoden." Zeitschr. wiss. Zool., Bd. xv. 1865. Nautilus, p. 203, 

 figures on Taf. xix. and xx. Later notes have been furnished by Haller, op. cit. 1895. 



3 Fraisse, P., "Ueber Molluskenaugen mit embryonalem Typus." Zeitschr. wiss. Zool. Bd. 35, pp. 461 — 477, 

 Taf. 25—26. 



