iv INTRODUCTION. 



general law. So common, however, is it, and so ready of 

 discernment, that it will probably be retained, even after 

 a more perfect, but less readily detective, system of 

 natural arrangement be discovered. 



The term was at first applied so as to embrace all 

 Crustacea that were not contained in the Stalk-eyed 

 order, with the exception of the Cirripedia. It is still so 

 retained in Mr. Dana's " Classification of Crustacea," and 

 consequently embraces a large number of forms^ exclu- 

 sive of those described in this Avork, which vary so con- 

 siderably from each other, that we believe it is neither 

 natural nor desirable to group them under one definition. 

 In the present volumes, we speak of the Sessile-eyed 

 Crustacea as constituting a legion between the Stalk-eyed 

 (Podophthalma) and the Entomostracous Crustacea. But 

 the great difference of character in some animals of this 

 legion from the others induced Latreille to divide it into 

 two orders, naming them respectively after the structure 

 of their locomotive appendages, Amphipoda and Isopoda. 

 Another division was proposed by the same author, and 

 very generally adopted, namely, the Lamipoda, or Lcemodi- 

 poda. The animals that constituted this supposed order 

 differ from the normal species of the Amphipoda only in 

 the absence and deficiency of parts ; consequently, in this 

 work, they are viewed as an aberrant group of the order ; 

 whereas Latreille first placed the animals of this group 

 in the order Isopoda,* and Lamarck united them with the 

 Amphipoda and Isopoda as members of one family only, 

 under the name of Arthrocephales, or Capites. Dumeril, 

 in his " Zoologie Analytique,'' united the Amphipoda with 

 the Stomapoda, the point of similarity being the sepa- 

 ration of the head from the body. 



The term Tdradecapoda has been proposed for the 



* Dictionnaire d'Hist. Nat. 



