278 GENERAL ZOOLOGY 



throw them upon the beach, where the soft portion rots and 

 dries. Then the remainder, which is the skeleton, is cleaned 

 and made ready for the market. These sponges are valuable 

 for the uses of man on account of the softness of the whole 

 skeleton and the tenacity of its substance. The skeleton is 

 composed of tough, anastomosing fibers of horny material 

 instead of spicules of silica, as in Heteromeyenia, or of car- 

 bonate of lime, as in Sycon. 



Relation to Environment. Perhaps no group of animals has 

 so wide a distribution in water as sponges. In fresh water, 

 and in the sea from the very margin of low tide to the greatest 

 depth of ocean yet explored, and in all zones, various species 

 of sponges are found. They live in every conceivable situa- 

 tion, adapting themselves in form of mass to the particular 

 place in which they grow. Branched species, like Microci'ona 

 prolifera^ in the frontispiece, vary much in form and arrange- 

 ment of branch. Incrusting species on rocks, as in the fron- 

 tispiece, and as shown growing about barnacles, Fig. 77, 

 follow every turn of the supporting substance. One species 

 of Clio'na grows on shells of mollusks, and through the 

 agency of a secretion of its protoplasm consumes the sub- 

 stance of the shell and grows to fill the space thus made. 

 In regions where the ocean-bottom is muddy, sponges grow 

 stalks that keep the mass away from the mud, which if 

 stirred up would smother the colony. 



Many marine sponges being thick and massive, and of loose 

 texture, like the sulphur sponges, are very convenient har- 

 bors of refuge for myriads of small animals, chiefly Crustacea 

 and worms. Undoubtedly the odor of living sponges, described 

 by one investigator as resembling garlic, drives away fishes 

 and other ravenous animals of large size that might feed on 

 the little guests, or even on the sponge itself. 



Definition of Porifera (Lat. porns, pore ; ferre, to bear). 

 By most investigators the sponges are considered a distinct 



