2 6 SEA AND LAND 



what like the shrinii)s. hcoan by adhering by their head-parts 

 to lloatiiiL; timber or rocks not much exposed to the waves, 

 and i^radiially, by one change after another, all apparently 

 clesignetl to the one end, have come to a nearly perfect recon- 

 ciliation with the conditions which surround them. Idieir 

 original forni is no longer recognizable, for they are now 

 cased in a cone formed of stony plates, and only these parts 

 fairl\- ancliored to the rock on which they rest. Their net- 

 like fringe of arms can, whenever for a moment the sea is still, 

 sweep the water about them ; and when the surge is about to 

 strike, withdrawing in their shells, which by their shape part 

 the; wax'e, they arc perfectly protected. So, too, the limpets 

 have converted the ordinary snail-like shell into a stout buck- 

 ler, which, when lifted as the w^ave withdraws, admits the sea- 

 water with its nutriment. As the water closes down on it the 

 edge of the shield comes upon the surface of the rock, and is 

 held there b\' the strong muscle which forms a large part of 

 the animal's body. Animals and plants pay with infinite toil 

 and pains for their chance to secure food in places where they 

 arc; fairly protected against organic enemies. The surf line 

 is by its conditions the best provisioned part of the sea ; it is 

 free from creatures which can prey upon its inhabitants ; and 

 to gain a place there, it is w^ortli while for any creature to 

 make many sacrifices. 



While the effect of this organic lift:, l)oth animal and 

 vecretable, is mainlv protective, b\- fendinir off the frost, and 

 to a certain extent diminishing chemical decay, there are 

 certain animals which themselves assail the rocks and, in a 

 measure, hast(;n their destruction. A whole group of shell- 

 fish related to our common mytilus, the sea-mussels of the 

 vernacular, are known as lithodomes or rock-house makers. 



