CRUSTACEANS 



419 



of a number of pieces fused together. The appearance of the 

 body within the cup has no doubt suggested the name "acorn "- 

 barnacle, but the resemblance is remote. 



Order 2.— BIVALVE CRUSTACEA (Ostracoda) 



These are small Crustacea common both in salt and fresh 

 water, especially where the bottom is muddy. Examination of 

 the mud from almost any pond will often reveal the presence of 



Fig. 260. — Small Fresh- water Crustacea (much enlarged to various scales) 



A, Water-Flea [Daphnia) : i, antennule; 2, antenna: 3, mandible: 5-9, flattened thoracic appendages. 

 &, Cyclops {seen from above): ant.i, antennule: ant.2, antenna. B', Swimming-foot of Cyclops, 

 showing the typical forked shape, c, Mussel-Shrimp {Cypris): ant. i, antennule; ant.2, antenna; 

 MND., mandible: mx.i, first maxilla; mx.2, second maxilla. 



one or more Mussel-Shrimps, species of the typical genus Cypris 

 (fig. 260), which therefore furnishes a convenient type. The most 

 striking feature is the presence of a bivalve shell, comparable to 

 the carapace of other forms and entirely enclosing the body, 

 reminding one of the arrangement characteristic of bivalve 

 Molluscs (p. 311). The resemblance is enhanced by the fact 

 that the two valves of the shell can be closed by the contraction 



