|MadSci Network: Zoology|
Daphnia do not have contractile vacuoles. Contractile vacuoles are pretty much limited to some protists and sponges. Daphnia, being crustaceans in the Phylum Arthropoda (Kingdom Animalia), have a complex, kidney-like organ built upon basic units called metanephridia. I did a simple search on "daphnia osmoregulation" and came up with the following URL which has some good, basic information on osmoregulation in animals in general.
Hope this has been useful.
Try the links in the MadSci Library for more information on Zoology.