CHAPTER XVIII. 



CRUSTACEA 1 . 



History of the larval forms 2 . 



THE larval forms of the Crustacea appear to have more faithfully 

 preserved their primitive characters than those of almost any other 

 group. 



BRANCHIOPODA. 



The Branchiopoda, comprising under that term the Phyllopoda 

 and Cladocera, contain the Crustacea with the maximum number 

 of segments and the least differentiation of the separate appen- 

 dages. 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. 



The free larval stages when such exist commence with a larval 

 form known as the Nauplius. 



The term Nauplius was applied by O. F. Muller to certain larval 

 forms of the Copepoda (fig. 229) in the belief that they were adult. 



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

 chapter. 



I. 



Branchiopoda. 



ii. Malacostraca. 



( Phyllopoda. 

 ( Cladocera. 



j-NebaliadiB. 



Schizopoda. 

 j Decapoda. 

 I Stomatopoda. 

 I Cumaceffi. 

 lEdriophthalmata. V. 



in. Copepoda. 



IV. Cirripedia. 



Ostracoda. 



JEucopepoda 

 I Branchiura 



/Thoracica. 

 \ A.bdominalia. 



tcint '' cl 



lApoda. 



u 



ilhizocephala. 



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. 



