CRUSTACEA. 213 



abdomen. Now although it is true there is not a single 

 known species in which all these segments are found in a 

 distinct and tangible condition^ there being in all the forms 

 more or fewer of them so inseparably united together as to 

 offer no otlier means by which to predicate their existence, 

 than those already alluded to, — yet, on the other hand, there 

 is not one which may not be found distinctly formed in some 

 or other of the species. The appendages too, which have 

 been already slightly mentioned, are no less subject to the 

 most extraordinary variation both of form and of&ce ; many 

 of them serving in one case the purposes of locomotion ; in 

 another, the reception and preparation of food ; in another, 

 the attachment of the branchia3 ; in another, the support and 

 protection of the eggs. When therefore we consider the 

 almost endless diversity of form under which the species 

 composing this class of animals appear, the astonishing dis- 

 crepancy which exists in the forms and relative proportions 

 of the diiferent regions of the body, and the other parts of 

 their organization, for the performance of offices equally 

 various, and see that all these diversities are produced only 

 by modifications of the typical number of parts, we cannot 

 but be struck by so remarkable and interesting an illustra- 

 tion of the great economical law, as it may be termed, that 



