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INTRODUCTION. 
The Ostracoda form a well-defined division of the Entomostraca. 
Like the Lamellibranchiata 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, 
(Cypris) 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. 
Linneus (or Linné) included all of the lower Crustacea in his genus 
Monoculus. 
+ O. F. Muller divided the lower Crustacea into several genera. Col- 
lectively they were called the ‘‘EHntomostraca.”’ 
Latreille and Cuvier 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 Branchiopoda and the FPoecilopoda. The first division 
included the Phyllopoda and Lophyropoda, while the Ostracoda fell into 
the second division. 
Following in the fvotsteps 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 Branchiopoda, he united them in the group Entomostraca. 
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 Copepoda 
