Tentacles q/Nautilus pompilius. 173 



and distinctly. The line does not extend throu2;h the 

 ejndermis or the nerve-cord. The lines are only found in the 

 upper portions of the- tentacles. It is difficult to understand 

 of what nse this arrangement can be. The tips of the 

 tentacles break off with exceeding ease ; but can this be in 

 any way advantageous to the Nautilus? 



The other point is regarding the structure of the nerve- 

 trunk. There is here what I have termed an accessory nerves 

 trunk. The usual nerve-trunk is present, having its layer of 

 ganglionic cells around its periphery and its ganglionic en- 

 largements in each segment of the tentacle. On the inner 

 side of this, through nearly the entire length of the tentacle, 

 runs a large nerve-trunk, composed of several bundles of 

 nerve-fibres. The main nerve-trunk and the accessory are 

 closely united, but are easily distinguished by the layer of 

 ganglion-cells which surrounds the main trunk. There are 

 very few, if any, ganglion-cells in the accessory trunk. At 

 the ends of the tentacle the accessory trunk gradually dis- 

 appears, at the upper end by giving off nerves chiefly to the 

 inner side of the tentacle, at the base of the tentacle by 

 gradual union with the main nerve-trunk. 



This accessory trunk has apparently been developed in 

 connexion with the remarkable sensitiveness of the ocular 

 tentacles. 



The nerves of the two ocular tentacles of each side are 

 branches of a nerve which comes off from the pedal ganglion 

 near the outer end, which also sends branches to the hood. 



The hood consists of the fused and enormously enlarged 

 sheaths of the dorsal digital tentacle of each side. The origin 

 of the nerves of the ocular tentacles iu the pedal ganglia, and 

 the fact that they form portions of nerves going to the sheaths 

 of digital tentacles, proves, as Dr. Willey has suggested, that 

 the ocular tentacles cainiot be considered as other than some- 

 what modified (and perhaps displaced) digital tentacles, and 

 that they can in no wise be considered as the homologues of 

 the optic tentacles and rhinophores of Gastropods. 



There is in the female Nautilus, ventral to the buccal mass, 

 a fleshy lobe, which, dividing into two near its tip, bears 

 upon each half ten to fourteen tentacles, and at the point of 

 division a rounded organ composed of a number of triangular 

 lamellae. This lobe (the inferior labial of Owen) is wanting 

 in the male Nautilus ; instead is found, nearly hidden beneath 

 the buccal mass, a rounded organ, which is named, from its 

 discoverer. Van cler Hoeven's organ. I think that anatomical 

 evidence is strong enough to convince us that the inferior 



