CRUSTACEA. 243 



Ostracoda have the exoskeleton formed into a pair of lateral 

 shells resembling those of bivalve M.ollusca. They show 

 a very degenerate condition of the body. Cypris is a 

 common freshwater type. The Copepoda are an immense 

 assemblage of marine and freshwater crustaceans, usually 

 of small size. They play the part in marine life of the 

 insects on land. Great numbers are pelagic and form the 

 staple food of larval fish. Cyclops is a common little "water- 

 flea" found in ponds. Many Copepoda are parasites and are 

 so modified in form and shape that their crustacean affinities 

 would hardly be recognised except for the early develop- 

 ment. Much the same remark applies to the Cirripedia^ 

 of which the barnacles and acorn-shells are important types. 

 The barnacle \Lepas) has a long stalk which is usually 

 ;affixed to a floating log, the hull of a ship, &c. The body 



Pig. 159. — Lateral View of Fig. 160. — Lateral View of 

 Lepas (Barnacle). (Natural size.) Lepas Anatifera. 



{Ad fiat.) {Adnat.) 



Scutu 



^|^^>fN\, 



Penis. 



.Tergum. 

 Carina. 



With right shell removed showing anima 

 Sj;alk. lying in mantle cavity. 



s .enclosed in five calcareous shells, and there are six pairs 

 )f legs which are covered with processes. Their perpetual 

 ovement serves to supply the animal with microscopic 

 id. The acorn-barnacle has no stalk and is enclosed in 

 conical outer shell in addition to the movable shells, 

 n each case the young start life as naupUus larvae, and 

 ass through the stage of a free-swimming crustacean which 

 xes itself to a foreign body and becomes a sessile adult. 



