206 THE ECONOMIC 1 IMPORTANCE OF ANIMALS 



Sponges. - The sponges of commerce have the skeleton composed of 

 tough fibers of material somewhat like that of cow's horn. This fiber is 

 elastic and has the power of absorbing water. In a living state, the 

 horny fiber sponge is a dark-colored fleshy mass, usually found attached to 

 rocks. The warm waters of the Mediterranean Sea and the West Indies 

 furnish most of our sponges. The sponges are pulled up from their resting 

 place on the bottom, by means of long-handled rakes operated by men in 

 boats or are secured by divers. They are then spread out on the shore in 

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

 consisting of beating, bleaching, and trimming, the bath sponge is ready 



for the market. Some 

 forms of coral are of com- 

 mercial value. The red 

 coral of the Mediter- 

 ranean Sea is the best 

 example. 



Pearls and Mother of 

 Pearl. - - Pearls are prized 

 the world over. It is a 

 well-known fact that even 

 in this country pearls of 

 some value are sometimes 

 found within the shells of 

 the fresh-water mussel 

 and the oj^ster. Most of 

 the finest, however, come 

 from the waters around 

 Ceylon. If a pearl is cut open and examined carefully, 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 at a given point, thus stimulating 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, 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 is working out a means of 

 artificial propagation of these animals. 



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 as is shown in 

 the picture. 



