242 MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS. 
dorsal tentacles, but in the adult animal are not visible (with the exception of the 
case of a single species, as far as we know,) from the exterior. 
When the skin is removed, they are seen as minute black dots, placed at the outer 
sides of the supra-oesophageal ganglia. They are thereto attached by a minute pedicle 
of variable length, which is so short in some as to make the eyes appear sessile on the 
ganglia. On closer examination the pedicle is found to have at its base a roundish 
or oval ganglion, which we have in a former part of this paper called the optic. 
The optic nerve emanating in a forward direction from this ganglion enters the base 
of a well-formed eyeball, consisting of, first, a delicate transparent investing membrane, 
within which rests a pretty regularly formed cup of black pigmentary matter, the 
choroid, having projecting from its mouth a globular, bright, crystalline lens, in front 
of which is a firm, transparent, well-arched membrane attached to the lips of the cup 
of pigment, the cornea, or perhaps the capsule of the lens. The eye of Doris is fully 
equal in development to that of Eolis ; but in the former, as it cannot be seen through 
the skin, we infer that there is no perception of external objects, but that at most the 
creature can only distinguish between light and darkness. This appears to be the 
necessary extent of visual power for the preservation from external violence of the 
individual in animals of such limited locomotive powers, the tactile property of the 
oral tentacles, assisted by the lips, possibly the seat of taste, being all that is requisite 
for the selection of matters fitted for the sustentation of the organism. 
The organs of Smell . — The dorsal tentacles, which have never been observed to be 
used as tactile organs, we believe to be the seat of the sense of smell ; and this belief 
is strengthened when we reflect that these organs are most highly developed and 
minutely laminated ; that they are most plentifully supplied with nerves from ganglia 
placed in front of all the rest of the cerebral masses ; that they are externally covered 
with vibratile cilia, and so placed on the head as easily to receive impressions from 
any odorous particles that may be mingled with the circumambient water. 
It is generally admitted now that snails have the sense of smell ; and Doris, which 
is certainly not inferior to them in organization, can scarcely be denied the possession 
of that endowment, particularly as we find in it a highly developed, conspicuous, sensi- 
tive and therefore important organ, to which no other use can properly be assigned, 
but which appears to correspond, in arrangement of parts and position, to the lami- 
nated antennae of insects, to which olfaction has been commonly attributed. In the 
Pearly Nautilus, certain laminae within the oral sheath, plentifully supplied with 
ganglia and nerves, have been by Professor Owen pointed out as the olfactory organ 
in that mollusk. It must be borne in mind that the laminated form of the organ of 
smell, and its supply of ganglia and nerves from the very front of the cerebro-spinal 
nervous centres, are universal in Fishes and the higher Vertebrata. In Fishes the 
organ presents, as is well known, a beautiful doubly laminated arrangement, the stem 
bearing the laminse being fixed longitudinally to the bottom of the cavity of the nose. 
