Closely related to the hermit crabs is the famous cocoanut crab or Birgos of the Spice Islands. This crab is so strong and powerful that, as Professor Van Beneden states, one clinging to a tree, seized a small goat and lifted it from the ground by the ears. The Birgos resembles a huge hermit crab, but has no artificial shell, the soft abdomen being protected by a shell of its own. This large land crab lives mainly on cocoanuts, which it secures by climbing the trees and biting off the stems. Descending, the crab will take the nut and with remarkable discrimination hold it with one claw and with the other tear off the husk, always at the end containing the "eyes." This stripping process, impossible to man without some implement, is remarkable in itself, and tells the story of the muscular strength of the crab. When the "eyes" of the nut are exposed, the crab seizes it by inserting its claws in the holes, and hammers the shell until it is broken. The crabs live at the base of the trees and line their dens with the husk. [Illustration: FIG. 153.--Horseshoe crab: _A_, upper side; _B_, lower side.] In ancient times crablike creatures existed, eight or nine feet in length. These are represented to-day by the quaint horseshoe or king crabs (Fig. 153). They are found in shallow water in Northern waters, and resemble a horseshoe with a long, sharp spike or spine--the tail of this strange animal. XVII. LUMINOUS CRABS One of the interesting experiences of Nordenskiöld in the Arctic Ocean was wading through the sludge, as the soft snow water along the beach is called, and seeing each footprint turn into a mass of light, caused by the phosphorescence of a small crustacean called Metridia. The light was bluish white, of great intensity, and although at times the cold was so severe that mercury would freeze, yet everywhere this marvelous light blazed. Even drops and splashes of the water seemed to be molten metal, but were merely alive with this minute light giver resembling Cyclops. In the Pacific, especially in summer, the exhibition of what might be called "crab light" is marvelous, and this is often true in the Atlantic. The light following the splash of an oar, the spray hurled aside by the cut water, the foaming water around a propeller, and the strange shifting specter which follows the rudder, are caused more or less by minute crustaceans which have the faculty of emitting light without heat.