1. About your test organism..
Daphnia magna!

2. About Daphnia magna:
Some Cladocerans:
It is a species of Daphnia (a freshwater water flea) which is native to northern and western North America. It is also widely distributed in Eurasia and in some regions of Africa.
It is in the Order Cladocera, along with many other small crustaceans. Cladocera has around 620 species that have been recognized so far, with many more undescribed.
They are very common in inland aquatic habitats, but rare in the oceans.

3. Anatomy:

4. Life Cycle:
This diagram shows the sexual and the asexual (parthenogenetic) life cycle of a Daphnia.
During the parthenogenetic cycle, females produce diploid eggs that develop directly into daughters. The same female may produce diploid asexual eggs that develop into sons. Male production is under environmental control.
During the sexual cycle, the same female may produce haploid eggs that require fertilization by males. These eggs are then enclosed in a protective pouch (ephippia) and need to undergo a diapause before female offspring will hatch from them.

5. Use in Toxicity Testing:
Domesticated, Daphnia magna is widely used as a laboratory animal for testing ecotoxicity starting with Einar Naumann in 1934.
The use of Daphnia magna as an experimental animal for such purposes is advantageous because:
Daphnids are small, so that a great many can be reared in a small space.
They have a relatively short life span, which reaches a maximum of about two months when they are reared at 25°C.
Daphnids are easy to culture, requiring only water containing bacteria or their equivalent for food. They can be grown individually in small bottles or in mass culture in large aquaria.
They mature early, giving birth to young within their first week of life. Each female who lives to a ripe old age can bear four hundred or more offspring.
Further, daphnids are representatives of a class of animals that serve as food for many fish, especially while the fish are young.