The framework is secreted in the mesoderm, and in different [pg102] genera consists respectively of a horny or silicious or calcareous substance, or of the first two of these substances combined. The sponge of commerce has the first kind and is composed entirely of exceedingly fine flexible fibers of a horny substance called /spongin/. In other species the spongin is intermixed with spicules of silica, or of carbonate of lime, in various shapes. In the sponges, so much valued as curiosities, called "Venus's flower-basket" and "glass-rope sponge," the framework is composed of silicious spicules alone. [Illustration: Various forms of sponge-spicules.] The spicules have a great variety of shapes, being rod-like, knobbed, three-pointed, six-pointed, anchor-like, etc., and are a feature in the classification of sponges. [Illustration: To illustrate choanocytes (/Ch/): section of a calcareous sponge. /Ect/, ectoderm; /Mes/, mesoderm; /N/, calcareous spicule; /Eiz/, ovum.] The sponge is traversed throughout by a canal system, consisting of a series of tubes through which water circulates, carrying air and food to the animal. The exterior of the sponge has numerous small pores and a comparatively few large openings. The fine pores are inhalent, taking in and straining the water of its coarser floating material, and then passing it through perforations in their sides into sacs lined with peculiar cylindrical cells having flagellate hairs, each hair having a collar at its base. These cells, called /choanocytes/, resemble independent animals of the /Protozoa/, known as /flagellate Infusoria/ or /Choanoflagellata/. [pg103] They take in and digest food and eject excrement from the area inclosed by the collar. The cilia (hairs) by their constant movement create currents which keep the water in motion. Water, then, is taken through the pores into the first or incurrent canals; thence it is passed into the ciliated chambers, and thence into the excurrent canals, and out through large passages terminating in large openings called /oscula/, or craters. The canal systems vary. In some species they become quite complex. [Illustration: Canal system of /Sycon gelatinosum/. Transverse section through the wall of a cylinder (parallel with the course of the canals), showing one incurrent canal (/IC/) and one radial (/R/) throughout their length; /sp/, triradiate spicules; /sp´/, oxeate spicules of dermal cortex (/dc/); /sp´´/, tetraradiate spicules of gastral cortex (/gc/); /ec/, ectoderm; /en/, endoderm; /pm/, pore-membrane; /pp/, prosopyle; /ap/, apopyle; /di/, diaphragm; /exc/, excurrent passage; /PG/, paragastric cavity; /em/, early embryo; /em´/, late embryo. (The arrows indicate the course of the water through the sponge.)]