NO EYES, AND MULTIPLE EYES 211 
Olauconidce. The members of these families 
can be distinguished the one from the other by 
the arrangement of their teeth which, in the 
case of the former, are restricted to the upper 
jaw, and in the latter to the lower one. 
Their eyes are extremely small, and almost, 
if not quite, hidden beneath the scales of the 
head. All of them are worm-like creatures of 
small size, and in distribution are to be found in 
the warm regions of the world. They spend 
the greater part of their time underground, 
only coming above the surface after rain. 
Their food consists chiefly of worms, although 
millipedes, ants and the larvae of various 
insects also form a part of their diet. 
Of the lower types of animals that are blind 
we have familiar examples in the earth-worms. 
The majority of the bivalves, that include such 
well-known forms as the cockles, the mussels 
and the oysters, are also unable to see, although 
a few of them, on the other hand, such as the 
bhorny-oysters (Spondylidce), and the scallop- 
shells or pectens , are provided with a number of 
eyes that are arranged in rows along the margin 
of their mantles. 
The thorny-oysters, found attached to the 
branches of growing coral, are further curious 
in the fact that when they attain to a certain 
age their shell ceases to grow externally but 
