164 THE HALL OF SHELLS. 
upon which the eyes are located can be drawn 
down through the tubes, as the finger of a 
glove is drawn in. The tubes are so trans-. 
parent one is able to see the black eye as it 
descends through the shortening tube to the 
little case under the skin of the head where it 
is safe, 
“These little tentacles or eye stalks are so 
sensitive of danger that the least impression is 
telegraphed through the nervous organism of 
the animal, and in an instant the eyes are out 
of sight, and if danger still seems imminent, the 
cautious little creature glides quickly into his 
fortress, 
“As proof of the clear vision of snails it is 
affirmed that they will go around obstacles in 
their path, and they appear to be attracted by 
bright colours. 
“ Among bivalves tiny specks have been 
detected near the borders of the mantle, some 
shining like jewels, others too small to be per- 
ceived except by a most careful scrutiny with 
a strong lens. These are supposed with good 
reason to be eyes. It has been observed that 
a sea urchin will sometimes turn its spines as 
if for self-defense in the direction in which a 
hand approaches to capture it. 
“The razor fish, even when buried in the 
