60 THE OPEN BOOK OF NATURE 
like appearance, the five arms raying out from the 
central disc. The mouth is in the centre of the 
body on the under side. The animal possesses a 
leathery skin in which are imbedded plates and 
spines of carbonate of lime. It has tube feet, which 
are little projections in a groove of the under side of 
each arm. ‘These feet are filled with a fluid, which 
is regulated by little sacs, or bladders; when the 
bladder connected with each foot contracts, it forces 
fluid into the foot and makes it extend. The feet 
have ends like suckers, and it is by their means that 
Fic. 11.—OpntoperMA EceErtont (4 Fossin, BrRiTtLE-STaR). 
the animal crawls. It catches prey with its arms, 
and eats it by extending its stomach out of its mouth 
and enfolding it—surely a curious way of feeding. 
There are also the Brittle-stars, near relations of the 
ordinary starfish. They have long thin arms, but 
they do not use their tube feet in crawling. They 
push themselves forward by four of their arms, and, 
as it were, feel their way with the fifth, which is 
pushed out in a forward direction. They are called 
‘* brittle-stars ’ because when they are caught by 
any of their arms, they break away from their 
