The Corals and various other animal organisms commonly known as "Zoophytes," forming together with the Jellyfishes the group Coelentera, are very effectively protected against the attacks of most predatory animals by the possession of "stinging cells," and this protection is shared by many other animals which shelter among them. Thus, the branching Coral stocks which grow in great luxuriance on tropical coasts support a rich and varied assemblage of animals, some of which may actually prey upon the Coral polypes, but all of which profit by the fact that few enemies venture to pursue them in their retreats. Innumerable prawn-like animals of the Alpheidæ and other families, and many kinds of Crabs, are found among living Corals. The Crabs of the family Trapeziidæ are especially characteristic of such habitats, and their thin, flat bodies seem to be adapted to slip into slits and crannies of the Coral blocks. The most highly specialized of all Coral Crabs, however, are the species of the family Hapalocarcinidæ, which modify in various ways the growth of the corals on which they live. In some of the more delicately branched kinds of Coral there may sometimes be found hollow bulbous growths, each of which contains imprisoned within it a little Crab--_Hapalocarcinus marsupialis_ (Fig. 66). It seems that the female Crab (the habits of the male are not definitely known) settles down among the branches of the Coral, and that the irritation of its presence causes the branches to grow up and surround it, coalescing with each other to form a kind of cage, and ultimately leaving only one or two small openings. Through these openings water can enter to enable the Crab to breathe, and no doubt food particles find their way in, but it is not possible for the Crab to leave its prison. The production of these abnormal growths of the Coral is closely analogous to the formation of "galls" on plants as a result of the irritation set up by the presence of insect larvæ or other parasites, and it is not inappropriate, therefore, to speak of them as "Coral galls." [Illustration: FIG. 66--TWO BRANCHES OF A CORAL (_Seriatopora_) SHOWING "GALLS" INHABITED BY THE CRAB _Hapalocarcinus marsupialis_. ON THE RIGHT THE FEMALE CRAB, EXTRACTED FROM THE GALL AND FURTHER ENLARGED] The Medusæ, or Jellyfishes, like other Coelentera, are provided with poisonous stinging cells, which, in the larger species of our own seas, are powerful enough to cause discomfort to bathers who come in contact with them. The protection thus afforded is no doubt of advantage to the little globular Amphipods of the genus _Hyperia_ (Fig. 67), which are almost always to be found sheltering under the bells of the larger Medusæ. In what way the Amphipods escape injury from the stinging cells of their host is not known. [Illustration: FIG. 67--_Hyperia galba_, FEMALE. ENLARGED. (After Sars.)]