Beginners' zoology . 20 BEGINNERS ZOOLOGY. spongin, a durable substance of the same chemical nature as silk(Figs. 30 and 31). The limy sponges have skeletons made of numerous spicules oflime. The three-rayed spicule is the commonest form. The commercial sponge, seen as it grows in the ocean, appearsas a roundish mass with a smooth, dark exterior, and having aboutthe consistency of beef liver. Several large openings (oscula),from which the water flows, are visible on the upper holes (inhalent pores — many of them so small as to beindistinguishable) are on the sides. If the spong


Beginners' zoology . 20 BEGINNERS ZOOLOGY. spongin, a durable substance of the same chemical nature as silk(Figs. 30 and 31). The limy sponges have skeletons made of numerous spicules oflime. The three-rayed spicule is the commonest form. The commercial sponge, seen as it grows in the ocean, appearsas a roundish mass with a smooth, dark exterior, and having aboutthe consistency of beef liver. Several large openings (oscula),from which the water flows, are visible on the upper holes (inhalent pores — many of them so small as to beindistinguishable) are on the sides. If the sponge is disturbed, the smaller holes, andperhaps the largerones, will close. The outer layer ofcells serves as a sortof skin. Since somuch of the spongeis in contact withwater, most of thecells do their ownbreathing, or absorp-tion of oxygen and giving off of carbon dioxide. Nutriment ispassed on from the surface cells to nourish the rest of the body. Reproduction. — Egg-cells and sperm-cells are produced bycertain cells along the canals


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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1921