OSTRACODA. 389 



bivalve shell (Fig. 256). In the marine Ostracoda the develop- 

 ment is simplified so that the metamorphosis is entirely absent, 



The Ostracoda feed partly on animal 

 and partly on vegetable matter. 



Numerous fossil Ostracods are known 

 from almost all formations, but only 

 the remains of their shells are pre- 

 served. They abounded in the Silurian 

 seas and attained there a much larger 

 size (90 mm.) than any known existing 

 genera. The deep-sea expeditions of FIG. 256. very young larva of 



Cypris (after Claus). Nauplius 

 recent years have however brought tO stage,with three pairs of appen- 



dages. A', A" first and second 



OUr knowledge the large free-SWimming antennae ; D intestine ; M 

 . . . stomach; Mf mandible;. 



Cypndinid Gigantocypris (q.v.), which SM shell muscle. 

 attains a length of 23 mm. 



Ostracods resemble one or other of the groups of Phyllopod^ 

 in the following features : the bivalve shells closed by an 

 adductor muscle ; the saccular heart, situated in front of the 

 point of reflection of the shell ; the presence of compound eyes 

 in the Cypridinidae ; the large setose plates attached to some 

 of the postoral appendages ; the large upper lip of the nauplius 

 larva and the retarded development of the second maxillae. 



On the other hand the highly specialized condition of the 

 mouth parts of the adult Phyllopoda the absence of the 

 mandibular palp and the reduced maxillae finds no parallel 

 in the Ostracods, for in these the mandibular palp is more leg -like 

 in character than in any other group of Crustacea, and the same 

 is true, though in a less degree, of both maxillae. In association 

 with these apparently primitive features in the Ostracods, we 

 may bear in mind the leg-like character of the second anten- 

 nae, and in the Myodocopa, of the mandibles, though these 

 may very possibly be adaptive features. The presence of a 

 bivalve shell in the nauplius larva is another feature in which 

 the two groups stand contrasted. 



Sub-order 1. MYODOCOPA.* 



The shell is notched in front to give play to the antennae. 

 The basal joint of the second antenna is usually wide, and the 



* The name Myodocopa (fj.vudtjs muscular, K^TTTJ an oar) is given by Sars 

 in allusion to the expanded muscular base of the second antenna, which 



