ARTHROPODA — CRUSTACEA 303 



Other examples of the Phyllopoda are Protocaris (Lower 

 Cambrian, looking much like Apus), Branchipus (living). 



Sub-class 3, Ostracoda 



Minute, with body short, unsegmented or very indistinctly 

 so, completely inclosed by the carapace which here becomes a 

 lateral, two-valved chitinous or strongly calcareous shell, usually 

 .5-5 mm. long ; this opens by means of an elastic, dorsal ligament 

 and is closed by a large, subcentral adductor muscle. When 

 closed the edges of the usually unequal valves as a rule fit closely, 

 the edge of the one valve within a groove at the margin of the 

 other. There is a median eye and at times also compound eyes. 

 When the shell is strongly calcified so as to become opaque, more 

 transparent spots in the antero-dorsal part of the shell mark 

 the position of the eyes. The position of the adductor muscle 

 is likewise usually noticeable upon the exterior of the shell as 

 a group of spots. There are seven pairs of appendages which 

 can be protruded when the valves open. The first and second 

 pair of antennae are large and are used for swimming or creep- 

 ing. The outer surface of the valves is seldom smooth ; usually 

 it is covered wdth rigid hairs, or is pitted or sculptured. Some- 

 times the shell is notched anteriorly to allow the passage of the 

 antennae when the valves are closed. The posterior half of the 

 shell is usually thicker than the anterior. The ostracod shell 

 is distinguished from that of the pelecypod by its minute size, 

 its usually unequal valves, its external sculpturing, the absence 

 of concentric growth lines, and the presence of the eye spot on the 

 exterior of the shell. 



Ostracods are abundant in marine, brackish and fresh waters, 

 usually creeping among the plants or burrowing in the mud. 

 They are abundant in all ages from the Ordovician to the pres- 

 ent ; usually only the bivalve shell is preserved in the fossil. 



Derivation of name. — Greek ostrakon, sl shell, 4- eidos 

 (oid), form ; the carapace takes the form of a two-valved shell. 



Cypris (living) is very abundant in stagnant pools. 



