30 LIFE AND HER CHILDREN. 
shells into which they can draw back entirely if they 
are attacked, but the flint-builders seem very careless 
in this respect, for they have large holes all over their 
flinty skeletons. But then, on the other hand, notice 
Flint Shells built by slime animals.* 
Immensely magnified : the real sizes are from a mere speck to that of 
one of these letters. 
how they send out sharp spikes, which must be un- 
comfortable for any animal trying to snap at them, 
although as we have seen (p. 16) the soft thread- 
* Radiolarije or Polycistinse. a, Petalospyris ; l>, Ethmosphoera ; 
r, Diploconus ; d, Dictyopodium ; f, Heliosphoera ; f, Actinomma. 
