158 



zooLoor 



some verjr abundant, minute, Ijean-shaped little crustaceans, 

 which have to move their appendages very vigorously to sup- 

 port their heavj' bodies in the water. The Ostracods are found 

 in almost all pools and streams, especially in the early spring. 



Many of them seem to be ex- 

 clusivelj^ parthenogenetic. 



Of the Copep'odai the 

 commonest fresh- water 

 genus is Cyclops, which 

 occurs in a similar habitat 

 with Daphnia and is some- 

 times found even in pure 

 drinking water. The fe- 

 male carries a conspicuous 

 egg-sac on each side of the 

 abdomen, and reproduc- 

 D a f) h n i a tion occurs with suchrapid- 



FiG. 1.5.5. — Daphnia. ant. 1 , 2, drst and ity that One Cyclops might, 

 second antcnnce ; hr, brain ; br.p, brood- i . i . r- i i 



pouch; d.ff/, gland;/, spines on the feet ; Under the mo.st tavorable 



W, heart; sA.ff. shell gland. After Claus. conditions, have 5 biUion 



descendants in one year. It is consequently easy to understand 

 how Cyclops often becomes the most abundant cntomostracan 

 in our waters, and how in some lakes it has Ijcen found that 

 there are over one million of them to each square metre of 

 water surface. Large numljers of the Copepoda are marine. 

 One of the most common is Acartia (Fig. 1.56), which swarms 

 to such an extent on the surface of the water as to make great 

 phosphorescent areas. 



Barnacles are the only attached non-parasitic Crustacea. 

 Certain species of them are found fastened to rocks on the 



^ kope, oar ; puu^, foot. 



