94 COMPARISONS OF STRUCTURE IN ANIMALS. 



Crustacea^ (lobsters, etc.,) arachiida, (spiders,) 

 and insecta, (true insects.) All, however, which 

 we can here say is, that in these classes, the 

 limbs are modified for almost every conceivable 

 purpose, for running, leaping, climbing, swim- 

 ming, burrowing, and even striking. In the 

 Crustacea they vary greatly in form and size; 

 in the lobster and its allied forms, they are 

 divided into three sets. On each side of the 

 mouth are six so-called limbs, termed jaw-feet, 

 furnished with filamentous appendages as organs 

 of touch. The use of these limbs is in the 

 manipulation of food, and its application to the 

 jaws. The true limbs are fixed under the 

 chest, and are five on each side. The first 

 pair are very large, and terminate in formidable 

 pincers, acted upon by powerful muscles. In 

 some Crustacea the pincers are alike on each 

 side, but not so in the lobster, the left being 

 finely dentated, for the purpose of cutting ; 

 the right being bluntly tuberculated for seizing 

 and holding fast. The pincers, both in form, 

 use, and magnitude, vary very greatly in the 

 Crustacea, and in some groups lose the character 

 of pincers altogether ; the same observation 

 applies to the form and development of the 

 other limbs, which present incessant variation. 



