At the foot of the arrangement of phyla in the metazoa stand the Porifera, or sponges, fixed, plantlike, queerly shaped beings living in the sea, except one family in fresh waters, and abundant in all the warmer parts of the world on rocky bottoms. Whatever its size or shape, a live sponge (of which the commercial article is the more or less perfect skeleton) is coated with a thin fleshy membrane perforated by minute "inhalant pores" and larger holes termed "oscula," or mouths. Through the inhalant pores the sea water, with its burden of microscopic food, enters one of many spaces beneath the surface from which incurrent canals penetrate the interior of the sponge, constantly branching and growing smaller until lost to sight. The fine tips communicate with small cavities lined with cells that are fitted to seize and assimilate the nourishment brought them by the water. From these rudimentary stomachs go similar excurrent ducts that unite near the surface into trunk canals that carry out the used water and waste products. This system of circulation, bringing nutrient water strained through the pores, and expelling it forcibly after it has been cleared of food value, is kept in motion, with occasional periods of rest, by the action of "flagellate cells" that line certain tracts in the canals. These are elongated cells from which project whiplashlike filaments, one to each cell, whose movements in concert "resemble those which a very supple fishing rod is made to undergo in the act of casting a long line"--the movement being much swifter from without inward. Beneath the outer skin, and all among the canals and cavities, is a filling of gelatinous materials, largely protoplasm, in which are formed great numbers of variously branched and strengthening spicules, of limy material in one group, and in others of a flinty or glassy nature, or in the absence of these, a network of "spongin," such as forms the skeleton of our common washing sponges. Spongin is a substance allied to silk in chemical composition, and the threads are felted together in such a way as to form a firm, yet elastic structure. "In some Noncalcarea, which are devoid of spicules, the place of these is taken by foreign bodies--shells of Radiolaria, grains of sand, or spicules from other sponges. In others again, such as the Venus's flower basket (Euplectella), the glass-rope sponge (Hyalonema), and others, the skeleton consists throughout of siliceous spicules bound together by a siliceous cement."