322 MARINE INVERTEBRATES 
It will be observed from the examples given that the shell plates 
do not entirely cover the mantle, but that a considerable portion 
of the upper mantle surface is left exposed along the side of the 
animal. This exposed portion of the 
mantle is called the girdle. It is always 
very much thickened and is often ex- 
ceedingly tough. In some species it is 
covered by a great number of caleare- 
ous spicules or granules which are em- 
bedded in the leathery mantle tissue. 
Strangely enough, the Chitonide (a large 
family of the Polyplacophora) possess a 
great number of eyes scattered over the 
shell plates like minute dots. 
The members of this order are to be 
found in all parts of the world, but 
their greatest development is upon the 
west coast of America. In California 
the student of marine life has a splen- 
did opportunity to examine the chitons. 
They are generally to be found adhering 
to the under surface of stones about low- 
tide mark. They are very sluggish in 
their movements and may readily be re- 
moved from their resting-places by sud- 
denly thrusting a knife-bladé under the 
foot and prying them off. When thus 
detached they have a habit of rolling up 
up into a ball and so remaining indefi- 
nitely; to prevent this they should be 
placed at once in a jar of sea-water, 
where they will resume their natural ae 
position. For preservation as cabinet cn ctenitins 1, focty ment mi 
specimens, chitons should be tied down "edge; ™, mouth. 
between two boards or flat substances, to hold them firmly ex- 
tended until dried; otherwise they are apt to contract out of 
shape. 
Six or eight species are exceedingly common upon the west 
Chiton spinosus, dorsal view. 
