THE MANTLE- CO VERED ANIMALS. 
distant relations to venture into new hunting grounds 
and become land animals. 
Watch a periwinkle some day in his home among 
the rocks, and see him gently lift his shell, open his 
horny door (p, Fig. 43), and put out his head. He has 
two delicate tentacles to 
feel with, and just behind 
these on very short stalks 
are set two tiny but keen 
eyes, the nerves of which 
join the great nervous 
mass now for the first time 
chiefly centred in a head. 
The under part of his 
body is a flat crumpled 
disk or foot, as it is called, 
composed of muscles; and 
this when lengthened out 
first on one side and then 
on the other, draws him 
gently along, the under 
side being moistened from 
time to time by slime 
from a gland within. On Th e ins ; d eofaPeriwinkle.-^. 
account of this foot ^ Foot . OT> musc le for drawing 
being under the body, back into the shell ;g, spittle glands; 
the periwinkle and his ^SfjKV^SS 
companions are called ing to s, stomach ; r, rasp of teeth 
Stomach-footed (Gastero- foiled up; Jr branchiae or breath- 
ing gills, which, when the mantle 
Poda). So he moves On, j s folded back in its place, lie over 
but at the slightest alarm the throat ; a, anus ; o, ovary carry- 
he disappears as if by 
magic into his shell, drawing his horny door close be- 
