INTRODUCTION. v 



Sessile-eyed Crustacea by M. Blainville, in contradistinc- 

 tion to that of Decapoda : the one being defined by having 

 fourteen legs, the other having only ten. But this, upon 

 the most superficial examination, will be found to be the 

 most imperfect character, not only in usefulness, but also 

 in appearance. Not only all the Sto?napoda, but even 

 the Macrura, below the family of Palcemonida, possess 

 fourteen fully developed pediform limbs ; and even in the 

 Brachyura and Anomura, the anterior appendages that 

 protect and supply the mouth are legs altered for a 

 necessary purpose, and not really oral appendages; conse- 

 quently, the distinction in structure that the two separate 

 names would lead a student to expect, does not exist. 

 The only true Decapoda are Caprella and Anceus, and 

 these belong, in the present system of classification, to 

 the Tetradecapoda. 



The term Choristopoda, or separate-footed, has been 

 applied by Mr. Dana, who uses it as synonymous with 

 Tetradecapoda of Blainville and our term of Sessile-eyed, 

 over which it appears to possess no advantage, without 

 which it is unwise to add to the already too numerous list 

 of synonyms. Thus it will be perceived that, in our con- 

 sideration of the orders treated of in this work, we 

 consider that the Sessile-eyed Crustacea bear a nearer 

 structural affinity with the Stalk-eyed Crustacea than with 

 the Trilobita, Entomostraca, and Rotatoria, which Mr. 

 Dana unites into the one division under the term of 

 Sessile-eyed Crustacea. 



The classification that we have adopted nearly resembles 

 the system of arrangement adopted by Milne Edwards in 

 his " Histoire des Crustaces ;" but, in his classification, the 

 aberrant Amphipoda are admitted to a rank of equal im- 

 portance to that of the Amphipoda, whereas certain very 

 exceptional forms of Isopoda are only distinguished as a 

 separate family of Isopoda. 



