Sponges are reproduced both by budding in some form, which is an asexual way, and by the sexual method of eggs and male cells; these are formed in the same sponge, but rarely at the same time, and the early stages of development are passed in a brood-cell within the body of the parent sponge. Finally, the embryo escapes through one of the outgoing canals, swims about awhile, becomes thimble-shaped, and settling down, fastens itself by the closed end to some patch of mud, a rock, dead shell or seaweed, closes the open end of the "thimble," and proceeds to grow. Sponges do not appear to be eaten by fishes or anything else. Countless lower animals, such as marine worms, mollusks, and so forth, burrow into them, however, in search of shelter; and in reversal, certain small sponges, such as the cliona of our shores, burrow into the shells of mollusks, which explains the honeycombed appearance of many of the shells picked up on the beach. Sponges have a large part in that very interesting and widespread phase of marine life called "commensalism," in which two animals become intimately associated in a mutually beneficial way, and are thus spoken of as messmates. Some kinds of sponge are never found growing except on the backs or legs of certain crabs; the sponge conceals and protects the crab, while itself benefits by being carried from place to place, with constantly new changes of fresh water and food. This sort of partnership occurs in many different groups of marine animals. The capture and preparation of sponges for market employ thousands of men and boats in the eastern Mediterranean, whence the best are derived, and in the West Indies and Gulf of Mexico, where the sponges are of a coarser kind, and are gathered and prepared by rougher methods. They are taken commercially also in other seas, and frequently dredged from vast depths. CHAPTER V FLOWERS OF THE SEA JELLYFISHES, NAMESAKES OF THE FABLED MEDUSA The type and simplest form of that great division of aquatic, and almost exclusively marine, animals constituting the phylum Coelenterata, is the polyp. It consists of a soft-skinned body, typically cup-shaped, containing a baglike digestive cavity, or primitive stomach, open at the top, and surrounded by the soft mesenchyme. The open upper end is the mouth, which is usually encircled by few or many tentacles--hollow outgrowths from the wall of the tubular gullet. Currents of water are drawn in by waving cilia at one end of the slitlike mouth, and pass out as waste at the other side; they bring food and oxygen from which nourishment is absorbed by the cells of the wall of the stomach (endoderm). Certain outgrowths within the mesenchyme act as feeble muscles for lengthening and shortening the body and tentacles; but there are no blood vessels or excretory organs.