5g CRUSTACEA. 



The use of the hinder legs in these species is therefore additional 

 evidence of their degraded system. The structure is not a primal 

 idea of itself, hut a result of the same cause which has degraded the 

 seiiBes. and given the whole character to the species. 



In these observations we favour no monad theory: we simply en- 

 deavour to illustrate the general law or plan which the Infinite Creator 

 exhibits in his works. 



Tribe I. BRACHYURA. 



Before offering remarks on the special classification of the Brachy- 

 tira, it is important to enter upon some general considerations with 

 respect to the importance of different organs as a basis of classification. 



It has already been explained that no a priori reasoning can prove 

 satisfactory; for there must be a special study of the objects to be 

 classified, before the value of the characters exhibited, even by one of 

 the highest order of organs, can be accepted as of paramount impor- 

 tance. We have illustrated this on a preceding page, by alluding to 

 the great discrepancies that exist among the different departments of 

 Crustacea, as regards the organs of the vital functions. The nervous 

 system is evidently the highest in its influence upon the vital energies 

 of the species, and its characters afford the most striking distinctions 

 between the several grand divisions in Zoology. Yet, general struc- 

 ture and plan of embryological development have a more exalted 

 importance; and though no nerves may be detected in certain Ra- 

 diata, they are Radiata still, and are not thereby removed from other 

 species in which such nerves may be distinct. It might seem a priori 

 very improbable, that species in which there is but one thoracic gan- 

 glion with radiating nerves, should be intimately related to species in 

 which there are half a dozen or more ganglions, at intervals in a long 

 cord; yet the nerves, though so unlike, are found to be indicative of 

 only a narrow divergence, merely that which divides the Brachyura 

 and Macroura. The general relations of structure, as exhibited in 



