SMELL 
31 
There is naturally a tendency for olfactory organs to be 
developed at the front end of the body, 
where they can be most usefully em- 
ployed, and they are commonly to be 
found on the feelers of Arthropods. In 
the Crayfish, for example, the small first 
feelers (antennules) bear groups of flat- 
tened bristles which undoubtedly have 
to do with smell, and similar structures 
are present on the antennz of Millipedes 
and Insects (fig. 1037). 
Land-Snails and Slugs, among the 
Molluscs, are known to be endowed 
with a keen sense of smell. 
In the common Garden-Snail 
(Helix aspersa) some of the 
epidermic cells at the tips of 
the long eye-bearing tentacles 
are believed to minister to this 
function (fig. 1038), though 
experiments have been made 
which appear to show that 
olfactory cells are elsewhere 
present. 
Among aquatic Molluscs 
what is known as a water- 
testing organ (osphradium) is 
usually present in the neigh- 
bourhood of the breathing 
organs (fig. 1039). This is 
generally considered to be of 
olfactory nature. 
In Vertebrates the sense- 
cells related to smell form part 
of the lining of the cavities of 
Fig. 1038.—Tip of Optic Tentacle of 
Garden- Snail, in diagrammatic longitu- 
dinal section, enlarged. The tentacular 
nerve (Tent. N.) gives off an optic nerve 
(Op...) to the eye, and then expands into 
a ganglion (Gz.) which sends fibres to an 
olfactory patch (O//) of cells on the tip of 
the tentacle. 
the nose, and since these are 
developed as pits in the ex- 
ternal surface, such cells must 
necessarily be of ectodermic 
character. When the sense of 
Fig. 1039.—Diagram of a Comb-gilled Snail, seen from 
above. The roof of mantle-cavity and overlying shell sup- 
posed transparent 
1, Mouth; 2, brain ganglion; 24, nerve-cord connecting 
side ganglion (abovg) with foot ganglion (below); 3, one of 
the three ganglia on the twisted nerve-loop; 4, gill; 4, os- 
phradium; 5, opening of intestine; 6, heart in pericardium; 
8, a gland (purple-gland in Purpura); 9, siphon; 10, ro, foot; 
11, operculum. 
