ORDER II. 

 CRUSTACEA ANOMOBRANCHIATA. 



THE Macroura have been described as divided into two prominent 

 series, one, the Thalassina series, which ranges up to the Paguridea, 

 and another, the Shrimp or Caridoid series, whose highest grade is 

 presented in the Astacoidea. Each of these series passes, by succes- 

 sive steps of degradation, into the Anomobranchiates; consequently, 

 there are two corresponding series among the Anomobranchiates, 

 The first, or Thalassina series, includes the Squillte and their con- 

 geners; the second, or Caridoid series, embraces Mysis and other re- 

 lated genera. 



The degradations in the species, to which we here allude, are appa- 

 rent in several ways. 



1. The carapax in the Anomobranchiates is never covered either 

 side behind by the sides of the first abdominal segment, this pecu- 

 liarity indicating a looser and less perfected structure in the body. 



2. The two outer pairs of maxillipeds, and sometimes the three 

 pairs, are pediform, and often are not distinguishable from the follow- 

 ing pairs of true legs, results of the less concentrated nervous system. 



3. The branchiae beneath the carapax are wanting ; and there are 

 either branchiiform appendages hanging externally at the base of the 

 thoracic legs (Euphausidce) , or, in some cases, at the base of the abdo- 

 minal legs ( Cythince and Squilloidea] ; or else, there are no branchije, 

 the general surface of the body performing the function of aeration 

 (as in the Mysince, Sceletinince, and Luciferidce) . 



