Messrs, Hancock and Embleton on the Anatomy of EKolis. 195 
organs pointed out by Professor Owen in his memoir on the 
Pearly Nautilus, which “ consist of series of soft membranous 
. laminz compactly arranged in a longitudinal direction, and situ- 
ated at the entry of the mouth, between the internal 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 
laminz, arranged in a manner not widely different from the dis- 
position of those of Eolis ; they are disposed, as in the Doridide, 
in a pinnate manner, attached to a central stem: examples of 
this may be seen in the dace and in the burn trout, Pl. 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 Holis the laminez 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 
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 Holis, we feel justified in 
assigning to them the office 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 
im some instances defended from external mechanical injury by 
a fence of delicate processes, as in Dendronotus arborescens, 
Pl. 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* 
