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 Stomapoda, but even 
the Muacrura, below the family of Palemonide, 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. 
