THE ECONOMIC BIOLOGY OF INVERTEBRATES 469 



tiny skeletons have produced great layers of chalk and other rock, 

 which form important land areas such as the Dover cliffs in southern 

 England. Some of the pyramids are made of stone formed from 

 protozoan deposits. 



Many protozoa perform valuable service as scavengers, and, 

 since they are mostly aquatic, aid in keeping our water supply 

 free from filth. On the other hand, a great many diseases are caused 

 by protozoa, among which are malaria, smallpox, yellow fever, 

 dysentery, scarlet fever, 

 etc. (See Chap. 13.) 



Sponges. From the next 

 higher group, the sponges, 

 man obtains the various 

 forms of the common 

 " sponge." The sponge is 

 really the horny skeleton 

 of the sponge animal, 

 from which the jelly-like 

 flesh has been removed by 

 rotting and washing. 

 Sponges grow attached to 

 the sea bottom in various 

 warm regions, such as the 

 Mediterranean and Red 

 Seas, and Florida and West 

 Indian waters. The best 

 come from the Mediter- 

 ranean. A live sponge is a roundish smooth mass, rather dark 

 brown in color, provided with many pores for passage of water, 

 and having about the consistency of a piece of beef liver. 



They are collected by divers or by dragging hooks, piled on 

 shore till the flesh rots off, washed, dried, sorted, and sometimes 

 bleached. The world's annual sponge crop is worth about 

 $4,000,000. 



Coelenterates. The coelenterates include many curious and 

 beautiful animals such as the hydras, hydroids, jelly-fish, corals, 



FIG. 149. Branching coral Acropora 



