SUB-KINGDOM II.
Spongida, or sponges, are especially worthy of mention. When much less was known of their nature and habits, they were classed with vegetables, but since their mode of reproduction has been discovered, they are known to be animals. The sponge is formed of an aggregation of ciliated cells, built together on a skeleton or framework of calcareous or silicious substance, that extends by slow external secretions as the animal body grows. There is a central cavity toward which there are numerous channels, from openings on the surface, through which water is continually received, and one through which it is discharged. The animal part is a sensitive, gelatinous film, extending through the growing mass, and spreads out over the surface. It is perforated, in places, and adapts itself to all the sinuosities and intersections of the canals that run in every direction. The little animals are provided with exceedingly fine cilia that they keep in almost constant motion; no one knows how, or for what, but it is supposed they thus sweep in the water that circulates through all the channels and chambers formed for it. After death, the soft matter, like all animal tissues, decays, or is dried up; and by beating and washing, it and any calcareous substances are removed. The horny, elastic fibers are found so exquisitely connected about the internal canals and cavities that water is freely admitted, or by pressure expelled. Sponges are found in every latitude, but are more numerous and grow larger in warm climates. Those in our markets are mostly from the Bahamas and the islands of the Mediterranean. They are obtained by diving, often to great depths.