ANISOPODA. 739 



the species are, to a great extent, very small, and have not been 

 generally sought for. 



The name of the tribe is from the Greek *, not, and Isopoda. To 

 most of the species, the name has also a literal application, as they 

 have commonly one or two pairs of anterior chelate legs, differing 

 from the following ; and, in some cases, as in Arcturus and Tanais, the 

 uses of the three hinder pairs and the four preceding are widely diffe- 

 rent. Thus the species are as strikingly Anisopodous or unequal- 

 footed, as the Isopoda are isopodaas or equal-footed. 



In arranging the Anisopoda, we should regard the striking paral- 

 lelism that exists between the several groups of genera, and those of 

 the Isopoda : thus, corresponding 



1. To the Idotceidea, there are Arcturus, Leachia, Anthura. 



2. To the Oniscoidea, there are Tanais, Apseudes, Rhcea, Bojoyris, 

 lone, and the allied. 



3. To the Cymothoidea, there are Serolis, Praniza, and Anceus. 



There are, therefore, three grand divisions, similar in character to 

 those of the Isopoda, as follows : — 



Subtribus I. Serolidea, vel Anisopoda Cymothoica. — Appendices abdo- 

 minis duae posticae lamellatae, apud abdominis latera dispositae. 



Subtribus II. Arcturidea, vel Anisopoda Idot^eica. — Appendices 

 abdominis duae posticae operculiformes, laminas branchiales tegentes. 



Subtribus III. Tanaidea vel Anisopoda Oniscica. — Appendices abdo- 

 minis duae posticae plus minusve styliformes, fere terminales, raro 

 obsoletae. 



Viewing the species with reference to their greater or less diver- 

 gence from the Isopodan type, we arrive essentially at the same 

 arrangement. This divergence is to be distinguished mainly in the 

 character of the abdominal appendages. In the first group, part or 

 all of the five anterior pairs of abdominal appendages are well-formed 

 branchial leaflets, as those of the CymothoidcB. In the second, the 

 same is equally true, and the first pair are opercula, like those of 

 the true Idotaeidae. In the third, the divergence is wider, the abdo- 



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