PKE-OOULAR AND POST-OCDLAR TENTACLES OP NAUTILUS. 199 



tentacles form deep grooves, between which the ridges project 

 as prominent lamellae. The upper and lower surfaces of the 

 lamellae and the bases of the grooves are covered with vibratile 

 cilia (fig. 2). There can be little doubt that the pre-ocular and 

 post-ocular tentacles of Nautilus represent tentacular pro- 

 cesseS; homologous with the adhesive tentacles, which have 

 been modified to serve an accessory olfactory function. We 

 will therefore speak of them as the olfactory tentacles, in 

 contrast to the adhesive tentacles. As is well known, there is 

 a rhinophore in Nautilus, placed directly below the eye, corre- 

 sponding to the rhinophore or olfactory groove of the Di- 

 branchiata. In Nautilus there is a small tentacle as well as a 

 fossa in connection with the rhinophore, but it is not annulated 

 and not retractile. 



The olfactory tentacles (apart from the rhinophore) when 

 extended stand out from the body nearly at a right angle, the 

 pre-ocular tentacle being directed slightly forwards, and the 

 post-ocular tentacle usually tending backwards (fig. 1). The 

 ciliated olfactory lamellae are directed strictly forwards. 



In the living Nautilus the olfactory tentacles otherwise offer 

 a strong contrast to the adhesive tentacles by their almost 

 uniform white colour. When examined under the microscope 

 there is found to be a little brown pigment in the annulations 

 and at the edges of the lamellae, but when viewed in to to 

 under water the general colour effect is white. 



Moreover the adhesive tentacles can be touched without 

 necessarily being retracted, but at the slightest contact with a 

 foreign body the olfactory tentacles are instantly retracted 

 within their sheaths. 



The presence of accessory olfactory tentacles in Nautilus 

 can, I think, be related to an essential bionomical difference 

 between the existing Tetrabranchiata and the Dibranchiata. 



Nautilus finds its food chiefly by the sense of smell, while 

 it is a matter of more or less common observation that the 

 Dibranchiata with their remarkably perfect eyes pursue their 

 quarry by the sense of sight. This difference, which is to a 

 certain extent evident from the facts of organisation, is further 



