Wired has a short blurb featuring some photos from Jack Challoner’s new book The Cell: A Visual Tour of the Building Block of Life, including the above Volvox picture. The caption is
Each green sphere is a colony of Volvox algae with more than 50,000 cells. Scientists study these glowing, freshwater organisms as models for how living creatures develop specialized cells and tissue. Strands of cytoplasm connect neighboring cells, allowing them to communicate, and slender flagella propel the colony through the water.