794 EYE ; RHINOPHORE ; OTOCYST. 



The eye of the limpet {Patella) resembles that of Nautilus upon a very small scale 

 and bears a similar relation to the tentacle (allowing for changes in proportion and in 

 topography) to that which the eye of Nautilus does to the rhinophore. The latter 

 remark applies with even greater force to the much larger eye of Haliotis (cf. my 

 PI. LXXVI. fig. 5), in which the cornea is likewise perforated, though the chamber of 

 the eye is filled by a vitreous body (Fraisse). 



The eye. of Pleurotomaria as described and figured by M. F. Woodward (op. cit. 

 1901) also resembles in principle the eye of Nautilus, the cornea being perforated, while 

 the cavity of the optic cup is only partially .filled by the vitreous body. 



In Nautilus the vitreous body is represented by a clear bacillary layer (Stabchen- 

 schicht) overlying the pigmented retinal epithelium, but it does not extend over the 

 inner surface of the cornea, which therefore appears as a black disc with the pupil in 

 the middle when seen from the inside (PI. LXXXIII. fig. 25). 



The extremely limited movements and accommodation of the eye of Nautilus suggest 

 that in spite of its size it is a photometric or photoscopic organ rather than a visual organ. 



The rhinophore was discovered by Valenciennes (1841). It consists of a small papil- 

 liform tentacle placed below the eye, with an aperture at its base leading into a 

 deep pit lined by a ciliated epithelium, which is thrown into low folds. From the depth 

 of the pit and the difficulty attending its dissection the designation olfactory labyrinth 

 seems to be justified (PI. LXXXI. fig. 6). 



The otocyst was discovered by Macdonald (1855). It lies over but not in the 

 posterior cornu of the cartilage and immediately external to the coronal ganglion, so 

 that it has to be turned aside in order to expose the latter from the outside (PI. 

 LXXXI. fig. 6, and PI. LXXXII. fig. 6). It contains in the fresh condition a milky 

 substance, which is so fluid that one experiences a certain difficulty in procuring some 

 of it for examination, so readily does it trickle away from the ruptured otocyst. Under 

 the microscope it resolves itself into a vast number of minute calcareous bodies called 

 otocones (PI. LXXXIII. fig. 2). These were correctly described by Macdonald, but some- 

 times the contents of the otocyst harden in alcohol into a coagulum which has oc- 

 casionally given rise to the erroneous impression that the otocyst of Nautilus contains 

 a single otolith. 



The eye and rhinophore of Nautilus apparently correspond with the eye and tentacle 

 of Haliotis or Strombus for example. Nevertheless this simple comparison has not always 

 been evident since Dr Pelseneer 1 in his work on the morphology of the arms of 

 Cephalopoda failed to grasp this relation, and apparently ignoring the existence of the 

 rhinophore 2 claimed the ophthalmic tentacles of Nautilus as the representatives of the 

 typical molluscan cephalic tentacles and stated that they are innervated by the supra- 

 oesophageal ganglion. 



1 Pelseneer, P., " Sur la valeur morphologique des bras et la composition du systeme nerveux central 

 des CMphalopodes." Arch. Biol. vm. 1888, see p. 730. 



2 Although he refers to the " nerf olfactif" in a footnote. 



