Animal diversity. Fig. 18-0a. Colonial protists as ancestors of modern animals. Fig. 18-2a. Somatic cells. Digestive cavity. Reproductive cells. Infolding. Colonial protist, an aggregate of identical cells. Hollow sphere of unspecial- ized cells. Beginning of cell specialization.
Most sponges have no symmetry, but the Choanocyte shown here has radial symmetry
Choanoflagellate colonies Sponge coanocytes Single choanoflagellate
Recent studies suggest that choanoflagellates are cousins to all animals in the same way that chimpanzees are cousins to humans. From left, a choanoflagellate colony, feeding cells (coanocytes) of sponges that resemble choanoflagellates and a choanoflagellate with its long flagellum and collar of filaments. There can be millions of choanoflagellates in a gallon of sea water. (Sean Carroll, New York Times, 12/13/2010)