This slide shows several stained specimens of the water flea Daphnia.  Called water fleas because of their jumpy movements, these crustaceans use their leaf-like second pair of antennae for respiration, locomotion and for filtering particulate matter from the water column (most species are filter feeders, although a few are carnivorous).   The bodies of most water fleas are enclosed in a bivalved carapace.  

Water fleas living in aquatic habitats subject to wide environmental fluctuations (such as drying up, completely freezing, crowding, etc.) show some particularly interesting reproductive adaptations.  During favorable periods, females produce diploid amictic eggs by parthenogenesis that turn into more females.  During unfavorable periods,  females produce some males as well as some mictic haploid eggs by meiosis that must be fertilized by males.  These eggs are highly resistant to desiccation, freezing, etc. and only hatch when conditions improve.