Animal Evolution. 197 



awkward, tiresome, and slow, not capable of much 

 improvement. 



Crustaceans. — Although the mechanism of crus- 

 taceans is wonderfully complete in its way, and 

 skilfully adapted to the peculiar mode of life mani- 

 fested by them, yet the sidelong motion of crabs 

 cannot be characterised a brilliant invention ; while 

 the plan of encasing animals in tight-fitting shells, 

 which growing obesity dooms them to desert, thus 

 endangering their lives each moult, can only be styled 

 a clumsy expedient, the tentative contrivance of a 

 blundering, mechanical apprentice. The connection 

 between the vegetal and animal kingdoms is 

 illustrated by crustaceans in that curious but useful 

 habit, unfortunately not extended to men, of repro- 

 ducing lost limbs. Thus crabs and lobsters may 

 often be seen disporting with ill-assorted old and 

 new claws. 



Star-fishes., sand-stars, and brittle-stars are all 

 endowed with the same convenient faculty of mending 

 themselves. Amputation, consequently, must be 

 comparatively painless among the lower organisms, 

 for brittle-stars especially are, exceedingly reckless 

 with their appendages, sometimes snapping themselves 

 into bits at the slightest provocation. 



Locomotion becomes more highly developed in the 

 leech. Here a shell is discarded, although some sea- 

 worms live in horny tubes. No external locomotive 

 organs are observable, and yet by merely lengthening 

 and shortening themselves, leeches move comparatively 



