CRUSTACEA. 243 
Ostracoda have the exoskeleton formed into a pair of lateral 
shells resembling those of bivalve Mollusca. They show 
a very degenerate condition of the body. Cyfris is a 
common freshwater type. The Cofepoda 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. Cyc/ops is a common little ‘‘water- 
flea” found in ponds. Many Cofefoda 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 (Zegas) has a long stalk which is usually 
affixed to a floating log, the hull of a ship, &c. The body 
Fig. 159.—LATERAL VIEW OF Fig. 160.—LATERAL VIEW OF 
Lepas (BARNACLE). (Natural size.) Lrepas ANATIIERA. 
(Ad nat.) (Ad nat.) 
Scutum. 3g Bens 
fe 
as 
4 5 
A Genital 
Aperture. 
4 
g 
q 
3 
Carina. 
With right shell removed showing anima 
lying in mantle cavity. 
is enclosed in five calcareous shells, and there are six pairs 
of legs which are covered with processes. Their perpetual 
movement serves to supply the animal with microscopic 
food. The acorn-barnacle has no stalk and is enclosed in 
a conical outer shell in addition to the movable shells. 
In each case the young start life as xaupus larve, and 
pass through the stage of a free-swimming crustacean which 
fixes itself to a foreign body and becomes a sessile adult. 
