lv 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 work, 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 Lemipoda, or Lemodi- 
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 Jsopoda,* and Lamarck united them with the 
Amphipoda and Isopoda as members of one family only, 
under the name of Arthrocephalés, or Capités. 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 Tetradecapoda has been proposed for the 
* Dictionnaire d’Hist. Nat. 
