RELATION TO OTHER ANIMALS 01 



these escape from the sponge. They settle down to 

 the bottom of the pond or stream and remain dormant 

 until the approach of warm weather, when they grow into 

 new sponges. They have a thick protecting coat which 

 enables them to resist unfavorable conditions. 



46. Economic Importance. — The spicules of the different 

 sponges form a large part of their so-called skeletons. 

 These spicules are, in some cases, composed of lime and 

 form the limy sponges. In others, they are of silica and 

 form the glass}?" sponges. The more important sponges 

 have a skeleton made up of a hornlike substance which is 

 flexible. This is the sponge of commerce. 



Great quantities of sponges are gathered from the sea 

 by divers and by dredges. The living tissues arc 

 allowed to decay, and the skeletons are then washed and 

 dried. Some are bleached to form the white sponges. 

 The sponges of best quality come from the Mediterranean 

 Sea and the Red Sea. 



Sometimes fresh-water sponges grow in the water mains 

 of cities and towns, causing the pipes to become clogged. 



47. Relation to Other Animals. — No animal is known to 

 eat the sponge. Sponges themselves feed on minute 

 particles of food, which are carried in by the currents of 

 water produced by the cilia of the endoderm. Some marine 

 animals use the porous body of the sponge as a retreat. 



Certain sponges live in close relationship to higher 

 forms of animals. One kind is always found growing on 

 the legs of crabs. The movement of the crab carries the 

 sponge to water richer in oxygen and food, and the crab 

 is hidden from its enemies by its sponge covering. Each 

 animal gains by this inter-relationship. Where two such 

 animals as the crab and sponge live in this way the rela- 

 tionship is known as symbiosis (sym-bi-o'sfe : Greek, syn, 

 with ; bios, life). 



