164 COMPARATIVE ANATOMY CHAP. 



The osphradial nerve usually springs from the pleuro- visceral connective (from 

 the parietal ganglion when this is present) ; in the Lamellibranchia it comes from 

 the parieto-visceral ganglion. The osphradial nerve is generally a lateral branch of 

 the branchial nerve. 



In the Lamellibranchia, the important fact has been demonstrated that, although 

 the osphradial nerve comes from the parieto-visceral ganglion, its fibres do not 

 actually rise from this ganglion ; but they pass along the pleuro-visceral connective 

 and have their roots in the cerebral ganglion. 



(b) Olfactory Tentacles. 



Certain experiments, to which, however, some exception might be 

 taken, seem to show that the large optic tentacles of terrestrial 

 Pulmonata are also olfactory. It is also generally accepted, though 

 still not certainly established, that the posterior or dorsal tentacles 

 (rhinophores) of the Opisthobranchia are olfactory organs. These 

 rhinophores (Fig. 93, p. 98) often show increase of surface, usually 

 in the shape of more or less numerous circular lamellae surrounding 

 the tentacle like a collar. The rhinophores are also often ear-shaped 

 or rolled up conically. Not infrequently they can be retracted into 

 special pits or sheaths. They are innervated from the cerebral 

 ganglion by means of a nerve which forms a ganglion at the base of 

 each. 



At the lateral and lower edge of the cephalic disc of the Cephalaspidce, an organ 

 which is considered to have arisen by the fusion of the labial and cephalic tentacles, 

 there are structures which are thought to be olfactory, and which, where most 

 developed, take the form of several parallel " olfactory lamellae " standing up on 

 the disc. 



(c) Olfactory Pits of the Cephalopoda. 



In the DibrancMa there is on each side, above the eye, a pit which 

 is considered to be olfactory. Its epithelial base consists of ciliated 

 and sensory cells, and underneath it lies, close to the optic ganglion, 

 an olfactory ganglion. The nerves running to this ganglion come 

 from the ganglion opticum, but really originate in the cerebral 

 ganglion. It looks as if these olfactory organs were the remains of 

 the posterior tentacles of the Gastropoda, and were comparable with 

 the rhinophores of the Opisthobranchia. In Nautilus the place of the 

 olfactory pit is occupied by the upper optic tentacle. We have 

 already seen that Nautilus still retains true osphradia. 



(d) The Pallial Sensory Organs of the Lamellibranchia. 



Several Asiphoniata have, in addition to the osphradia, epithelial 

 sensory organs, which lie on small folds or papillae to the right 

 and left of the anus, between it and the posterior end of the gill. 

 These are innervated by a branch of the posterior pallial nerve. 



Epithelial sensory organs of various forms (plates of sensory epithelium, sensory 

 lamella 4 , or papillae, tufts of small tentacles) are found on the mantle in tin- 



