190 Mr. A. Hancock on the Olfactory Apparatus in the Bullidse. 



in this higher type of form. This organ in B. hydatis resembles 

 in situation and general form that of Philine aperta ; but instead 

 of being a simple plate scarcely rising above the surface to which 

 it is attached, it is composed of a central stem bearing numerous, 

 considerably elevated, lateral laminse. This beautiful organ 

 escaped the observation of Cuvier, and appears to have been 

 generally overlooked by naturalists. Mr. Wm. Clark gives a 

 pretty full description of it in his paper on the Bullidce in the 

 6th vol. of the 'Annals,' 1850 ; but seems to have entirely mis- 

 understood its nature. He considers these " leaflets " salivary 

 glands, or inclines to believe them such. They are not, however, 

 glandular, are external, have no duct; and moreover well-cha- 

 racterized salivary glands exist in their normal position, supplied, 

 as they always are, with nerves from the buccal ganglions. The 

 fact that these leaflets receive their nerves from the olfactory 

 ganglions is alone sufficient to explain their nature. These 

 ganglions are of considerable volume, and are formed of nodulous 

 branches, which penetrating the skin divide into numerous twigs ; 

 these go to supply the laminse composing the leaflets. The an- 

 terior portion of this organ tapers into a stem-like prolongation, 

 which is lost, as in P. aperta, in the oral lip ; and hei'e, as in that 

 species, the prolongation is supplied with branches from the 

 labial nerve. This nerve comes from the anterior margin of the 

 cerebroid ganglion, and has in its course several ganglionic swell- 

 ings*. 



In this species, then, as well as in P. aperta and in Gastero- 

 pteron, taste and smell would appear to be connected. Be this 

 however as it may, it is impossible to look upon this interesting 

 modification of the olfactory organ in B. hydatis, and not see at 

 once its similarity to that of fishes, and to the dorsal tentacle of 

 Doris. It differs from that of the former only in not being in- 

 closed within a cavity, though it lies protected and concealed in 

 a groove ; and its transformation into the tentacle of the latter 

 can very easily be imagined. If the posterior portion of the 

 tentacular lobe were divided down the median line, and the por- 

 tions so divided raised up, the leaflets would then be seen on their 

 posterior surface. It would now only be requisite to extend the 

 laminse round the sides of the elevated portions, and they would 

 assume the appearance of the laminated tentacles of a nudibran- 

 chiate mollusk. 



* As this paper was passing through the press, I have ascertained that 

 Acera aplysiaformis is likewise supplied with an olfactory disc similar to 

 that of Philine aperta ; and what is of still more importance, that it has at 

 each side of the mouth a well specialized organ of taste, both the labial 

 nerves terminating in a bundle of filaments, the extremities of which pass 

 to the surface of the skin. 



