$22 ANIMAL REPRODUCTIONS. 
the mufcles of the fkin, the mouth, throat, and 
palate. 
As far as I know, Swammerdam was the firft 
who obferved that the brain of the {nail is move- 
able, and that its mobility arifes from fome muf- 
cles to which it is attached. By means of thefe 
it is drawn towards the fore part of the head, or 
is extended, according to the different motions 
of the body. When the animal ftretches con- 
fiderably from its fhell, the brain is commonly 
fituated above the cefophagus. 
It has already been pbferved, that the black 
point at the extremity of the horns is commonly 
thought the eye ; and fuch an opinion feems very 
reafonable. Indeed the fingular ability of the 
often named Dutch naturalift, and the affiftance 
of powerful magnifiers, was able to difcover the 
principal parts that characterize the organ of vi- 
fion; the uvea, the three humours, and the 
arachnoid, invefting the chryftalline lens. I 
have fucceeded in fufficiently diftinguifhing all 
thefe parts, except the aqueous and vitreous hu- 
mours, which it has never been in my power to 
difcern clearly ; but this I rather afcribe to my 
inability in examinations fo minute, than to the 
non-exiftence of the parts. Of the four nerves, 
proceeding from the brain to the horns, two, 
- 4 a 
which we fhall denominate the optic, are attached _ 
to the eyes, and enlarge into a kind of gourd or 
rather 
