[Illustration: FIG. 20.--a, Talitrus locusta, b, Orchestia littorea, the two common kinds of "sand-hopper." Of the natural size. c, A kind of small lobster which burrows in the sand, Callianassa subterranea. About two-thirds the natural size, linear.] In the open sea there are many hundreds of kinds of small crustaceans resembling the sand-hoppers in their compressed (not flattened) shape of body and in the details of their legs and the grouping of the joints of the body. Many of the smallest crustaceans which swarm in the surface waters of the sea and form part of that floating population, mostly of small transparent or iridescent and blue creatures, which we call the "plankton," or "surface-floating" population, and may be gathered by towing a very fine net behind a boat on a quiet day, can produce flashes of light which are vivid enough when seen at night. They contribute, together with jelly-fish and the teeming millions of minute bladder-like Noctiluca, and other unicellular animalcules, to produce that wonderful display seen from time to time on our coasts, and called "the phosphorescence of the sea." These minute crustaceans produce flashes of light by suddenly squeezing from pits or glands in the skin a secretion which is chemically acted on (probably oxidized) by the sea-water, the chemical action setting up light-vibrations, but not the usual excess of heat-vibrations to which we are accustomed when light accompanies ordinary "burning" or "combustion." [Illustration: FIG. 21.--A Phosphorescent Shrimp (Euphausia pellucida). The lamp-like phosphorescent organs are numbered 1 to 6. There is another on the outer edge of the stalked eye, making seven in all on each side of the animal. _g_, points to the hindermost gill, enlarged.]