_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. SUB-KINGDOM III. _Cœlenterata_ (hollow intestines). These are radiate animals, have a distinct digestive cavity, and always two layers of tissue in their walls. They have minute sacs containing a fluid, and barbed filaments capable of being thrown out as stings. The classes of the Cœlenterata are the _Hydrozoa_, _Anthozoa_, and _Ctenophora_. The best known representative of the former is the fresh water _hydra_ (water animal). It gives but slight evidence of a nervous system, has no stomach, or digestive sac, but is a simple tube into which the mouth opens. The sensitive little body is in color and texture, to the casual observer more like a plant than an animal. It is attached at one end to a submerged aquatic plant, while the mouth at the free end is provided with tentacles,[2] by which it feeds and moves. It buds, and also produces eggs. The young hydra, when hatched or thrown off, attaches itself to plants, as did its parents. Some hydroids attach themselves to shells, and are supported by horny, branching skeletons, specimens of which are numerous, and may be seen in almost any museum. [Illustration: HYDRA, SHOWING BUDS.]