In this respect--in having only one body cavity--sea-anemones and the coral-polyps and the jelly-fishes and the tiny freshwater polyp or hydra, and the marine compound branching polyps like it--agree with one another and differ from the vast majority of animals, such as worms, sea-urchins, star-fishes, whelks, mussels, crustaceans, insects, spiders and vertebrates (which last include fish, reptiles, birds, and mammals). These all have a second chamber, or body cavity, quite shut off from the digestive cavity and from the direct access of water and food particles. This second distinct chamber is filled with an animal fluid, the lymph, and is called the "Cœlom" (a Greek word meaning a cavity). These higher animals, which possess a cœlom as well as a gut, or digestive cavity, are called "Cœlomata," or "Cœlomocœla," in consequence; whilst the sea-anemones, polyps, and jelly-fish form a lower grade of animals devoid of cœlom, but having the one cavity, or gut, continued into all parts of the body. Hence they are called "Cœlentera," or "Enterocœla," words which mean that the cavity of their bodies (Greek _cœl_) is made by an extension of the gut, or digestive cavity (Greek _enteron_). The higher grade of animals--the Cœlomocœla--very usually have a vascular system, or blood-vessels and blood, as well as a cœlom and lymph, and quite independent of it; also some kind of kidneys, or renal excretory tubes. Neither of these are possessed by the sea-anemones and their allies--the Enterocœla--but they have, like higher animals, a nervous system and also large ovaries and spermaries on the walls of their single body cavity, which produce their reproductive germs. These pass to the exterior, usually through the mouth, but sometimes by rupture of the body wall. All "one-cavity" animals, the Enterocœla or Cœlentera, produce peculiar coiled-up threads in their skin in great quantity--many thousands--often upon special warts or knobs. These coiled-up threads lie each in a microscopic sac; they are very delicate and minute and carry a virulent poison, so that they are "stinging" threads. Excitement of the animal, or mere contact, causes the microscopic sac to burst, and the thread to be violently ejected. The sea-anemones, jelly-fish, and polyps feed on fresh living animals, small fish, shrimps, etc., and catch their prey by the use of these poisonous threads. Some jelly-fish have them big enough to act upon the human skin, and bathers are often badly stung by them. The commonest jelly-fish do not sting, but where they occur a few of the stinging sort are likely to occur also. Even some sea-anemones can sting one's hand with these stinging threads. One sea-anemone (known as "Cerianthus"), occasionally taken in British waters, makes for itself a leathery tube by the felting of its stinging threads, and lines its long burrow in the sand below tidal exposure in this way.