u6 LIFE AND HER CHILDREN. 
hind him, for the powerful muscles of his mantle 
(m, Fig. 44) enable him to shorten or lengthen his 
body at will. If undisturbed, however, he finds his 
way to a mass of seaweed, pushes out his snout 
(s, Fig. 43), and moves very slowly along, scraping 
fine shavings off the weed as he goes, so as to leave 
minute dents behind him. 
This he does by means of a very curious instru- 
ment. If you could look into his mouth, which 
opens on the under side of his head, you would find 
it paved with sharp teeth, just as if a number of 
nails had been driven into it point upwards, and it is 
with these that he rasps the seaweed as he rubs his 
jaw along it. 
But this rough file wears away rapidly with con- 
stant use, and to meet this difficulty he has a com- 
plete provision hidden within. The rasp within his 
mouth is only the end of 600 rows of teeth, three in 
a row, growing on a long gristly strap like pins stuck 
in a pincushion, and this strap, often two and a half 
inches long, closes its edges together at the back of 
the mouth so as to wrap over the rough points, 
and is then rolled, up into a coil, and stowed away 
in a fold of the neck (r, Fig. 44). As the front 
teeth wear away this strap comes gradually for- 
wards on the floor of the mouth, the new teeth 
grow up and are sharpened, ready for use. This 
curious strap is generally called the " tongue," 
though a " rasp " (radula) is a much more appro- 
priate name. 
And now as our periwinkle walks and feeds he 
must also breathe, and, strange as it may seem to 
us, no creature below the back-boned animals ever 
