68 THE CRUSTACEA 



Icnown to be parasitic, but one species found in the gill-chambers of 

 Crayfish in North America may be so. The majority are minute, 

 very many not exceeding 0"5 mm. in length. The Myodocopa are 

 usually larger than the Poclocopa, many being 1 or 2 mm. long. 

 The largest known Ostracod, a species of Gigantocypris, is 23 mm, 

 in length. 



Palaeontology. 



Fossil Ostracoda are abundant in all geological formations from 

 the oldest to the most recent, but, with hardly an exception, only 

 the shell is preserved and the affinities of the numerous genera 

 which have been established remain quite uncertain. Most of the 

 Ostracoda from Cambrian rocks l^elong to the genera Primitia, 

 Jones, and Lq)erditia, lioualt, while Bcyriclda, M'Coy, is common 

 in the Silurian. Species referred to the recent genus Cypridina 

 occur in the Carboniferous, and Cythere and other recent genera are 

 stated to make their ajDpearance in the Permian. 



Affinities and Classification. 



In the bivalved form of the shell, the ventrally flexed posterior 

 part of the body, the form of the caudal furca (in the Myodocopa), 

 and the biramous natatory antennae (especially of the Cladocopa 

 and Platycopa), the Ostracoda present important resemblances to 

 the Conchostraca and Cladocera among the Branchiopoda. There 

 might, indeed, be little difficulty in regarding them as derived from 

 the Conchostraca, were it not for the presence of the mandibular 

 palp, Avhich is absent from the Branchiopoda. The possession of 

 this apjjendage and its biramous form in many cases point to 

 the origin of the Ostracoda as an independent branch of the 

 primitive Crustacean stock. There appears to be no reason, 

 however, to suggest, as Claus has done, that the small number of 

 appendages is a primitive character. 



In the possession of paired eyes and a heart, the Cypridinidae 

 are more primitive than the other Ostracoda, although in the form 

 of the antennae and in some other points they are probably more 

 specialised than tlie Cladocopa and Platycopa. 



The classification given below is that of Sars, as modified by 

 Brady and Noi'man. In the more recent arrangement of G. AV. 

 Miiller the Cladocopa are included in the Myodocopa and the 

 Platycopa in the Podocopa, and the number of families is considerably 

 reduced. 



Sub-Class Ostracoda. 



Order 1. Myodocopa. 



Shell generally with antennal notch ; antennae Avitli massive basal 

 segment, generally biramous, exopodite generally with eight or nine 



