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Sponge. A group of low, many-celled, water animals forming the branch Porifera. The body is penetrated by numerous channels, some of which are inlet tubes, others outlet tubes, and a third set connecting the two. The outside of the sponge is covered with a thin, leathery membrane, the ectoderm; the pores are lined with a softer membrane, the endoderm, and between these is a third membrane, the mesoderm, which is strengthened by fibrous, limy, or glassy material. In the connccting tubes are chambers lined with ciliated cells. The water enters through the inlet tubes, passes through the connecting tubes, and is expelled through the outlet tubes. In passing through the chambers lined with ciliated cells, the cells capture and digest minute vegetable and animal organisms in the water, and by movement of the cilia create currents which keep the water moving through the sponge. The bath sponge of commerce is the siliceous skeleton of a sponge. Sponges are of almost universal occurrence and are almost exclusively marine, being generally found attached to the under side of projecting rocks, or clinging to the roofs of submarine caverns. The sponges of commerce are mostly obtained from the Grecian Archipelago, Florida, and the Bahamas.

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