SEA-CRABS. 179 



concealed among the seaweeds, it wages war with annelides, pla- 

 narias, and small molluscs requires no very active movements, 

 and they answer admirably either as holdfasts among the cliffs 

 and boulders, or for seizing their prey in the deeper crevices of 

 the rocks. 



In the Portuni or true Sea-crabs, finally, we find the hind pair 

 of legs flattened like oars, so that they would cut but a sorry 

 figure on land, but are all the better able to row about in their 

 congenial element. 



The comparatively short and weak though well-formed legs 

 of the lobster, and of the allied species of long-tailed decapods, 

 can evidently bear them along bj.it slowly when they attempt 

 to crawl. But the long flattened tail of these animals, ex- 

 panding laterally like a fin, serves, by its vertical strokes, to 

 propel them so rapidly through the water that the lobster 

 makes leaps of twenty feet at one single bound, and the 

 shrimp is seen to dart about in its native element with a 

 swiftness similar to that of the gnat or dragonfly in the lighter 

 atmosphere. 



The elongated hind-legs of the sandhoppers, contracted while 

 at rest, enable them, when suddenly extended, to emulate 

 the leap of the flea or the bound of the podura ; while the feet 

 of the Whale-lice, or Cyami, are armed with power- 

 ful claws, which are evidently necessary, to pre- 

 vent their being washed away during the rapid 

 evolutions of their enormous victims. In the terres- 

 trial species of the Oniscidse, popularly known by 

 the name of Wood-lice, the large number of the 

 feet makes up for their smalmess ; in the aquatic Whale Louse 

 or natatorial members of the family, the last pair (y amu3 Cet1 -) 

 of legs and the last shieldlike segment of the abdomen form a 

 large fin ; while the short feet of the parasitical species, such as 

 the Bopyrus Squillarum, which passes its life under the tail of 

 the shrimp, is provided with strong claws, for the purpose of 

 securing a firm attachment to their living prey. 



The digestive apparatus of the crustaceans is also most ad- 

 mirably modified, according to the respective wants of their 

 various tribes. Those which, in their state of perfect growth, 

 live almost invariably attached to their prey, without executing 

 any other motions than such as are performed by the latter, are 



N 2 



