INTRODUCTION. 



The Ostracoda form a well defioed division of the Entomosiraca. 

 Like the LameUibranchiata the body is always enclosed in a bivalve 

 shell, which is closed by powerful abductor muscles. No doubt the 

 casual observer often mistakes the Ostracoda for miniature mollusks. 

 Yet the resemblance is all on the surface; for, although the structure 

 of the enclosed Ostracodan is quite primitive, yet it is decidedly crus- 

 tacean. 



The body always bears seven pairs of articulated appendages. 

 The first five of these belong to the head; but, in different groups, 

 either or all of the last three of these cephalic appendages may be 

 modified for walking. The last two pairs of members belong to the 

 body and are properly called legs or feet. In some cases, however, 

 {Gijpris) the last pair of legs is never used for locomotion. In most 

 cases the abdomen bears a pair of appendages, which usually resemble 

 one-jointed legs. Morphologically they probably are legs which have 

 been arrested in their development. 



It is thought that a brief resume of the steps by which the Ostra- 

 codes have been assigned to their present taxonomic position may be 

 of interest. 



Linnaeus (or Linne) included all of the lower Crustacea in his genus 

 Monoculus. 



O. F. Miiller divided the lower Crustacea into several genera. Col- 

 lectively they were called the '■^ Entomosiraca.^^ 



Latreille and Cavier were the first to separate the Crustacea from 

 the Insecta. Basing their judgment upon the morphology of the 

 mouth parts, they divided the lower Crustacea into two artificial 

 groups, the Brancliiopoda and the Poecilopoda. The first division 

 included the Fhyllopoda and Lophijropoda, while the Ostracoda fell into 

 the second division. 



Following in the footsteps of Latreille, Milne-Edwards, with the 

 mouth parts for criteria, established an artificial system of classifica- 

 tion for the Crustacea. Separating the Copepoda and Ostracoda from 

 Latreille's Brancliiopoda, he united them in the group Entomosiraca. 



Dana modified slightly Latreille's system. 



The first attempt to form a natural system of classification for the 

 lower Crustacea was made by Zenker. He separated the Cox)ep)oda 



