Water Flea with eggs - Daphnia pulex - This most common species of water flea belongs to the family Daphiniidae. It has a cosmopolitan distribution in a wide range of aquatic habitats, although it is most closely associated with small, shaded pools and lakes of an oligotrophic nature. (Their range extends across most of the world - Americas, Europe & Australia - in appropriate habitats, most of which offer little by way of nutrients)
A Drosophila melanogaster embryo using in situ hybridization to visualize mRNA expression of the gap gene giant (red) and the pair-rule gene even-skipped (blue), with Sytox Green (Invitrogen) used to localize nuclei. Specimen preparation and imaging by Cecelia Miles, Dr. Martin Kreitman's lab; projection and cover design by Dr. Vytas Bindokas; University of Chicago, USA.
Volvox are green algae that form hollow, spherical colonies of thousands of cells. In some species, individual algae are connected by strands of cytoplasm, and there is division of labor between different groups of cells. Volvox and its relatives have therefore been used as model systems to study the evolution of multicellularity and cellular differentiation. More about these algae: http://eol.org/pages/11637/ Image by Wolfgang Bettighofer via micro*scope (cc-by-nc)