I. ON THE CLASSIFICATION OF CRUSTACEA. 



The following observations on the classification of Crustacea relate 

 only to the grander divisions of this class of animals. The various 

 subdivisions and their distinctions come under consideration in the 

 succeeding descriptive part of the work, in connexion with the 

 detailed descriptions of the several groups. 



In presenting our remarks on this subject, we offer first a few ob- 

 servations on the limits of the department of Crustacea, and a brief 

 review of the distinctive characters of the class. 



I. LIMITS OF THE DEPARTMENT OF CRUSTACEA, AND DISTINCTIVE 

 CHARACTERS OF THE CLASS. 



1. Limits.— The only point of doubt that has existed of late upon 

 the limits of the department of Crustacea refers to the lower orders of 

 the class, and, through recent investigations, the uncertainties are now 

 mostly removed. The Cirripedia have been claimed by the concho- 

 logist as Mollusca, and the Rotifers by Ehrenberg and others as a 

 branch of Infusoria. The former are so completely like Cypridinea 

 in the young state, as first shown by Thompson,* both in external 

 form and internal structure, and so unlike any species of Mollusca, 

 that their relations to the Articulata were made out satisfactorily by 

 this observation alone. The author collected some of these young 

 Anatifte in the Equatorial Atlantic in 1838, and, not being aware at 

 the time of Thompson's investigations, the species were referred to 

 the Cypris group. Subsequent investigations off Fuegia, where the 

 young and adult forms were found together, enabled him to trace out 

 the transitions. Another fact of importance, fixing the relations of 

 the Cirripeds, was observed by the author in the harbour of Rio 

 Janeiro. Numerous exuvi® of Cirripeds were collected, proving that 

 these animals undergo exuviation,— a process in growth characteristic 



* Zoological Researches, by J. V. Thompson, Esq., F.L.S., No. 3, January, 1830. 



