Mr. A. Hancock on the Olfactory Apparatus in the BuUidse. 189 



tentacular lobe as designated by the great French anatomist, is 

 largely developed, and the olfactory ganglions, the same as in 

 Doris, Eolis, and several other mollusks furnish nerves to the 

 dorsal tentacles, are not much less in size than the cerebroids 

 themselves. They each give off two or three nerves, which passing 

 backwards ramify on the skin of the under surface of the free 

 margin of the lobe, reaching even to the posterior extremity. 

 Hence it must be allowed that this part of the tentacular lobe 

 is the homologue of the dorsal tentacles of other mollusks ; and 

 there is, therefore, reason to believe, that as in the tentacles so 

 in this portion of the lobe resides the faculty of smell. The oral 

 tentacular nerve supplies the anterior portion of the lobe, parti- 

 cularly the overhanging margin, by which touch would appear 

 to be more decidedly exercised. 



Olfaction becomes more specialized in Philine aperta than in 

 Gasteropteron, and the olfactory ganglions are highly developed, 

 of a branched form inclining outwards towards the sides of the 

 head, and sending numerous twigs through the skin to the 

 under surface of the tentacular lobe, where dividing and subr 

 dividing they are lost in an oval plate, of nervous matter, of a 

 simple leaf-like form, which tapering forwards terminates in the 

 oral lip. This oval plate, then, is the seat of olfaction in P. 

 aperta. The stem-like continuation of it is, however, supplied 

 with branches from the labial nerve, which arises from a large 

 dendritic ganglion attached to the anterior margin of the cere- 

 broid. This portion of the plate, therefore, probably performs 

 some function in common with the lip — most likely that of taste ; 

 and if so, then in these mollusca the senses of smell and taste are 

 intimately connected : and in this there appears nothing strange, 

 when we reflect on the relationship that exists between these two 

 senses in the higher animals. In Gasteropteron the labial nerve 

 also sends branches to the under surface of the anterior portion 

 of the tentacular lobe. 



Akera hullata is hkewise furnished with an olfactory plate or 

 disc of an oval form, situated on the under surface of the ten- 

 tacular lobe; but in this species it is coarsely wrinkled trans- 

 versely, and its connexion with the lip is effected by a narrow 

 belt which passes from about the middle of its superior or outer 

 margin. The nervous element is here supplied from the same 

 source as in P. aperta ; only the labial and olfactory nerves have 

 no ganglionic swellings. 



It is, however, in Bulla hydatis that we see the olfactory organ 

 most highly developed. In this mollusk it has attained a struc- 

 ture perfectly similar to what is observed in fishes, while at the 

 same time it is a most instructive link, showing the analogy that 

 exists between the dorsal tentacle of Doris and the olfactory organ 



