SPONGES.
The sponge of every-day use (Figs. 11, 12) is composed of a horny, fibrous material, produced by a colony of Amoebae, and if a section of the fresh-water Sponge (Spongilla) be made, a glance will explain by what means the currents of water are produced, which can be seen under the microscope. The outer layer is composed of a number of Amoebae, with little openings, through which the water enters the cavity between the outer and inner layers, this inner layer being also composed of Amoebae, in the deep substance of which are chambers lined with fine hairs (cilia), which, working in the same direction, force the water through them into a common outlet; in this manner strong currents of water pass in and out of the sponge, making a little whirlpool, into which minute particles of matter are dragged.
Sponges are found attached to all kinds of rocks, and often to shells, both in the sea and on the beach, and are of two kinds, soft and hard, of which the soft are probably the ancestors of the hard. The Halisarca, or Slimy Sponge, is found attached to the leathery sea-weed, and is composed of slimy Amoeba-like bodies, in which the canal system just described is only imperfectly developed. From this kind were derived the Gummy Sponges, so called from their gumlike consistency; their canal system foreshadow-s the homologous structure in the Jelly-fish. Sponges like the Halisarca were probably the ancestors of the first kind of
