ANIMAL REPRODUCTIONS. 227 
than on the head and neck. The under part of 
the fnail is not fhagreened ; on the contrary, it 
is very {mooth and flippery. Some naturalifts 
call it the foot, and not improperly, from fup- 
porting the animal in its progreflion. This is all 
that the eye can perceive: we are obliged to ufe 
the f{calpel for penetrating the interior of the 
head, which is the objet of our refearch.- A 
{nail cannot be diffected alive: however much 
it is extended, it contracts entirely at the flighteft 
touch, and, retiring precipitately into its dwel- 
ling, lies in concealment there. On breaking 
the fhell for examination, the head and. horns 
are found retracted in fuch a manner within the 
body, as renders it extremely difficult to make 
obfervations on them conveniently. The eafieft 
method to follow is that propofed by the great 
Swammerdam, in his excellent treatife on {nails, 
which is killing them flowly in water: then they 
almoft always remain with the head and neck ex- 
tended from the thell, and the horns protruded. In 
fuch an advantageous fituation, when the fkin of 
the head is longitudinally divided with fine fharp 
{ciflars, there immediately appears the brain di- 
wided into two lobes; from the under part of 
which originates the medulla oblongata, and from 
above, the nerves: four are inferted in the four 
-horns, and extend to the extremities; the other 
~ fix divaricate to different parts of the body, as 
the 
