Messrs. Hancock and Emblcton on the Anatomy of Eolis. 195 



organs pointed out by Professor Owen in his memoir on the 

 Pearly Nautilus, which " consist of series of soft membranous 

 laminee compactly arranged in a longitudinal direction, and situ- 

 ated at the entry of the mouth, between the niternal labial pro- 

 cesses,^^ are similarly constructed, and also supplied with nervous 

 filaments from a pair of ganglia that are connected with the an- 

 terior cerebral or brain. 



In fishes the olfactory organ consists of delicate membranous 

 laminae, arranged in a manner not widely different from the dis- 

 position of those of Eolis ; they are disposed, as in the Dorididce, 

 in a pinnate mannei*, attached to a central stem : examples of 

 this may be seen in the dace and in the burn trout, PI. VI. fig. 12. 



In the higher Vertebrata the laminated form is evident 

 wherever we look. It may be objected to this argument, that 

 in the case of Eolis the laminse are arranged on the exterior of 

 the tentacle, and in the Vertebrata in the interior of a cavity ; 

 but if we can conceive of the tentacle of a Dendronotus, or of a 

 Doris coccinea, PL VI. fig. 11, retracted within a sheath, we have 

 then a very good representation of the olfactory organ of the 

 fish. 



Further, the ganglia of the tentacular nerves are in front of all 

 the rest, and are attached by their tractus to the anterior part of 

 the cerebral mass, — the anterior median ganglia, an arrange- 

 ment which, together with the anterior superior position of the 

 tentacles themselves, perfectly corresponds to that of the acknow- 

 ledged olfactory apparatus in fishes and all other Vertebrata. 



Lastly, if these tentacles be olfactory organs, we should expect, 

 in tracing downwards the animal scale, that they would disap- 

 pear before the tactile organs, the oral tentacles. That such is 

 the rule even in the Mollusca we have the authority of Professor 

 Owen. From what we have brought forward on this subject 

 respecting the anatomical details, the external configuration, and 

 the homology of the dorsal tentacles of Eolis, we feel justified in 

 assigning to them the ofiice of olfaction rather than in supposing 

 them to be the seat of some new and hitherto undescribed and 

 mysterious sense, or even of touch, as is generally believed. That 

 they are not for touch seems to be indicated in some measure by 

 their dorsal position, their direction upwards, and by their being 

 in some instances defended from external mechanical injury by 

 a fence of delicate processes, as in Dendronotus arborescens, 

 PI. VI. fig. 8. 



The sense of vision is subserved by two minute organs some- 

 what inferior in development to those of the higher Gasteropods. 

 They are situated beneath the skin, and are visible to the naked 

 eye as two black dots immediately behind the dorsal tentacles ; 

 they are each supported by what appears to be a short thickish 



13* 



