506 The Animal Kingdom 



of this subclass such as Eubranchipus do not have shells, others such 

 as Estheria have bivalve shells, while Apus has a peculiar thoracic shield. 

 Included in this subclass also are the cladocerans. These are small 

 fresh-water forms which are very abundant among the floating or- 

 ganisms. Most genera such as Daphnia possess a bivalve shell. 



The second subclass, Ostracoda, contains the ostracods. These are 

 small crustaceans which are encased in a bivalve shell. Species occur 

 in both fresh and salt waters, and they may be floating forms or live 

 in the bottom mud. Genera such as Cypris may become extremely 

 abundant. 



Fig. 166. — Some fresh-water crustaceans. A, Daphnia, a cladoceran; B, Cypris, 

 an ostracod; C, Cyclops, a copepod; D, Apus, a notostracan fairy shrimp; E, Eu- 

 branchipus, the anostracan fairy shrimp. 



Another subclass is the Copepoda. These are mostly small free- 

 living forms which occur in both fresh and salt waiter. Cyclops is a 

 common genus. This group contains some parasitic forms which live 

 on fish and often do considerable damage. Argulus, the fish louse, has 

 prominent suckers for attachment. Others, such as Lernaea, are re- 

 duced in the adult to simple egg sacs. 



The subclass Cirripedia contains the barnacles which economically 

 may be considered among the most important of the crustaceans. All are 

 marine forms and usually they are sessile. The common gooseneck barn- 

 acle, Lepas, has a stalk and an expanded distal portion enclosed in a 

 mantle and calcareous plate. The animal is so oriented inside the mantle 



