CHAPTER XVIII 



CRUSTACEA 1 . 



History of the larval forms'*. 



THE larval forms of the Crustacea appear to have more faith- 

 fully preserved their primitive characters than those of almost 

 any other group. 



BRANCHIOPODA. 



The Branchiopoda, comprising under that term the Phyllo- 

 poda and Cladocera, contain the Crustacea with the maximum 

 number of segments and the least differentiation of the separate 

 appendages. This and other considerations render it probable 

 that they are to be regarded as the most central group of the 

 Crustaceans, and as in many respects least modified from the 

 ancestral type from which all the groups have originated. 



1 The following is the classification of the Crustacea employed in the present 

 chapter. 



(Phyllopoda. \Natantia. 



I. Branchiopoda. jciadocera. III. Copepoda. * uc P e P oda \p ara sita. 



(Branchmra 

 " Nebaliadae. fThoracica. 



II. Malacostraca. i Q 



btomatopoda. iRhizocephala. 



Cumacese. V. OstraCOda. 



- Edriophthalmata. 



2 The importance of the larval history of the Crustacea, coupled with our compara- 

 tive ignorance of the formation of the layers, has rendered it necessary for me to 

 diverge somewhat from the general plan of the work, and to defer the account of the 

 formation of the layers till after that of the larval forms. 



