356 



ECONOMIC IMPORTANCE OF ANIMALS 



horn. This fiber is elastic and has the power to absorb water. In 

 a hving state, the horny fiber sponge is a dark-colored fleshy mass, 

 usually found attached to rocks. The warm waters of the Medi- 

 terranean Sea and the West Indies furnish most of our sponges. 

 The sponges are pulled up from their resting places on the bottom, 

 by means of long-handled rakes operated by men in boats, or they 

 are secured by divers. The}^ are then spread out on the shore in 

 the sun, and the living tissues allowed to decay ; then after treat- 

 ment consisting of beating, bleaching, and trimming, the bath 



sponge is ready for the 

 market. 



Some forms of coral 

 are of commercial 

 value. The red coral 

 of the Mediterranean 

 Sea is the best example. 

 Pearls and Mother 

 of Pearl. — Pearls are 

 prized the world over. 

 ]\Iost of the finest, how- 

 ever, come from che 

 oysters and clams in 

 the waters around 

 Ceylon. If a pearl is 

 cut open and examined carefull}^, it is found to be a deposit of the 

 mother-of-pearl layer of the shell around some central structure. 

 It has been believed that any foreign substance, as a grain of sand, 

 might irritate the mantle of the oyster at a given point, thus stim- 

 ulating it to secrete around the substance. It now seems likely 

 that most perfect pearls are due to the growth within the mantle 

 of the clam or oyster of certain parasites which are stages in the 

 development of a flukeworm. The irritation thus set up in the 

 tissue causes mother of pearl to be deposited around the source of 

 irritation, with the subsequent formation of a pearl. 



The pearl-button industry in this country is largely dependent 

 upon the fresh-water mussel, the shells of which are used. This 

 mussel is being so rapidly depleted that the national government 

 has worked out a means for its artificial propagation. 



In some countries little metal images of Buddha 

 are placed within the shells of living pearl oysters or 

 clams. Over these the mantle of the animal secretes 

 a layer of mother of pearl. 



