Text Appearing Before Image: 228 THE AMERICAN MUSEUM JOURNAL greatly in thickness, and the canal system may become much complicated through the folding of the walls of the paragastric cavity and the development of wide mesodermal cavities (JAih. and exh^. 3. The Skeleton. One of the most remarkable features of sponge structure is the skeleton. It is by far the most reliable basis for classifying the sponges yet discovered, inasmuch as it is comparatively unaffected by the external surroundings of the individual and Text Appearing After Image: FIG. 12. —A BAHAMAN COMMERCIAL SPONGE (//^>/-f/<'«^'<J J/. Hyatt) Showing colony of tube-like individuals therefore its peculiar features remain constant to the groups of which they are characteristic. It may be composed either of fibers or spicules, and it is secreted by the mesoderm. Its function is to furnish a rigid supporting framework for the body and to act as a protection against the enemies of the sponge. The fibrous sponges include among others those known to commerce. The skeleton is, in most cases, made up of interlacing and anastomosing fibers of a homy substance called spongin,
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