File:The American Museum journal (c1900-(1918)) (17972009618).jpg

Title: The American Museum journal
Identifier: americanmuseumjo06amer (find matches)
Year: c1900-(1918) (c190s)
Authors: American Museum of Natural History
Subjects: Natural history
Publisher: New York : American Museum of Natural History
Contributing Library: American Museum of Natural History Library
Digitizing Sponsor: Biodiversity Heritage Library

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A GUIDE TO THE SPONGES 239 closely akin to silk in chemical composition. It is secreted by the mesoderm and is arranged so as to be a supporting basis to the layers of cellular tissue composing the soft parts of the animal. The fibers are of two kinds,—first, a set of long stout principal ■fibers (see Fig. 11), from ^ to ; mm. in diameter, radiating from the base of the sponge to its surface, and secondly, a complicated network of fine connective fibers interlacing between the principal fibers and supported by them. The connective fibers are ex- tremely delicate, having a diameter of only .01 to .02 mm. and with meshes scarcely as large as their diameter. Grains of sand are often found imbedded in the principal fibers, in some cases forming a considerable part of the skeleton, in others the entire substance. The spongin fiber is made up of a soft central core
Text Appearing After Image:
FIG. 13—SOME TYPICAL FORMS OF SPONGE SPICULES or medullary axis, surrounded with successive layers of the spongin substance. The classification of the homy sponges is based upon the minute characters of the network. A few sponges of small size have no skeleton at all, being supported by whatever rigidity their tissues may possess, but with these exceptions all except the homy sponges have skeletons made up of spicules instead of fibers. These are small needle-like bodies composed of either carbonate of lime or silicon. The latter is found combined with water in such proportions as to form a substance chemically resembling opal, and of transparent glassy appearance. Hence spicular sponges may be classified as calcareous or silicious according to the nature of their skeletons. Spicules may have one or two axes, or their axes may radiate in 3, 4, 5, 6, or even 8 different directions, and are found in a great variety of forms,

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