MECHANISM OF RESPIRATION, ETC. 791 



teristic of Gastropoda and Pelec}i3oda (Lamellibranchs), Nautilus affords a most interesting 

 link between the Cephalopoda and these two groups. 



It appears that in the dibranchiate Cephalopoda there are no osphradia', and it 

 is probable that they have been lost in correlation with the much higher development 

 of the cerebro-visceral nervous system in these forms as compared with Nautilus. 



There are two pairs of osphradia in Nautilus, one to each gill. The fact of the ex- 

 istence of branchial sense-organs in Nautilus was first made known by Lankester and 

 Bourne in 1883, and the jmir described by them is that which corresponds with the 

 lesser gill on each side. It appears as a small arcuate papilla between the bases of the 

 gills ; the sensory surface is below the arch (cf PL LXXXIII. figs. 6 — 8). 



The second pair of osphradia appears as a quadrilobate papilla, often refeiTcd to as 

 the post-anal papilla, because it occurs behind the anus when the mantle is drawn back. 

 This compound papilla obviously consists of two bilobate papillae which have coalesced 

 in the middle line, though the degree of coalescence varies considerably, and occasionally 

 they are quite separate. In freshly captured specimens I soon became convinced of the 

 homodynamy^ of these papillae with the osphradial papillae of Lankester and Bourne, and 

 eventually recognised the fact that the deep fissure which causes the bilobate appearance 

 of the former corresponds with the angle made by the arch of the latter with the wall 

 of the mantle, and that in both cases the sensory epithelium was provided with vibratile 

 cilia, and was practically destitute of the gland-cells which are so abundant elsewhere 

 (PI. LXXXIII. figg. .3—8). 



In the light of the preceding facts, more especially after the discovery of cilia on 

 the protected surfaces of both pairs of osphradia, I decided in my own mind that the 

 osphradial nature of the "post-anal papilla" was established, and I had seen traces of 

 small nerves branching out towards the osphradia from the main visceral nerves (cf PI. 

 LXXVIII. fig. 1), though I had not ascertained their mode of termination in the sensory 

 epithelium. Pelseneer has recently denied that the "post-anal papilla" is iimervated 

 from the visceral nerves. He says^: " Qu'il n'y a pas non plus de fibres nerveuses allant 

 de ces nerfs a la papille postanale elle-meme, et que celle-ci n'est pas un organe sensoriel, 

 comme le supposait Willey. L'extr^mit^ de ces nerfs visc^raux medians innerve seulement 

 le manteau et, chez la femelle, la glande nidamentaire." 



Again on p. .58 of his work Pelseneer refers to the "Papilla postanale" in these 

 words: — "Ainsi qu'il a ^te dit plus haut, elle ne revolt pas de nerf important; elle ne 

 presente, ni sur elle ni autour d'elle, aucune region sensorielle. Son assimilation a une 



' The cephalic sense-organ consisting of a " small antero-posteriorly directed ridge, which is placed below 

 and a little behind each eye," described by Mr W. E. Hoyle as the "osphradium" in Guimlus fabricii 

 {P. Zool. Soc. London, 1889, p. 125), obviously corresponds with the rhinophore of Xautilus. The osphradium 

 is often or usually described as an olfactory organ, but the term is limited to the branchial sense-organs, to 

 which it was first applied by Professor Lankester (Article " Mollusca," Encijc. ISrit.). 



- This homodynamy was also indicated by Van der Hoevcn, although he was ignorant of their osphradial 

 nature. He says (Tr. Zool. Soc. London, Vol. iv. Part i. 1850) on p. 24 :—" Behind the auus [with reflected 

 mantle] there are on each side two small and depressed caruncles very similar to that mammillary eminence 

 or papilla we have seen at the root of the first branchia." 



2 Pelseneer, P., "Kecherches morphologiques et phylog^nfitiques sur les Mollusques arohaiques." Hem. Cotir. 

 Ac. Belgique, t. lvii. 1899, p. 57 (Reprint). 



