Figure 2.2. Compass jellyfish, Chrysaora hysoscella, from the spectacular 1904
book Kunstformen der Natur (Artforms of Nature) by Ernst Haeckel.
Figure 2.3. Planarian nervous system.The tiny nematode or roundworm Caenorhabditis elegans is an
animal that has, since the 1970s, been widely studied by biologists
interested in molecular, cellular, and developmental biology. It is
small (about 1 millimeter [1 mm] in length) and somewhat trans-