470 



BIOLOGY FOR BEGINNERS 



and sea-anemones, but the only forms directly of use to man are 

 the corals. Colonies of these tiny animals, called coral polyps, 

 secrete so much limestone in their body walls that they form the 

 coral reefs which make up large parts of several continents, notably 

 Australia and the Pacific islands. Other coral reefs of very ancient 

 times now form important beds of limestone like the " corniferous " 

 ledges that cross central New York. The red coral used for jewelry 

 is another product of this group, found principally in the Mediter- 

 ranean. 



Echinoderms. The echinoderms include the starfish, sea-urchin, 

 and sea-cucumber. Starfish are an enemy of the oyster and a 

 special effort is made to keep them out of oyster beds. The Chinese 

 and West Pacific peoples also use the sea-cucumbers for food, as 

 soup, and consider them a great delicacy. 



Worms. As already stated in our study of worms (Chapter 20), 

 we owe to the humble earthworm a heavy debt for his services in 



keeping the soil in fertile 

 condition; and we must not 

 forget that without this work 

 we should probably have 

 much difficulty with our 

 agriculture. On the other 

 hand, the parasitic worms, 

 such as tape-worm, hook- 

 worm, trichina, and other 

 intestinal forms, cause serious disease or death in man. Similar 

 forms, the flukes, infect our domestic animals, especially sheep, 

 which they attack by way of the liver and cause the death of 

 hundreds of thousands every year. 



Molluscs. Primitive man, before he knew the use of fire, de- 

 pended upon raw molluscs for much of his food, as the enormous 

 shell heaps remaining to this day testify. Even yet we look upon 

 oysters, clams, mussels, and scallops as useful foods or luxuries, 

 depending on how far we live from the seacoasts where they are 

 caught. In all, except the scallops, we eat the whole body, the 

 bulk of which consists of the liver and reproductive glands. What 



FIG. 150. Liver-fluke (Fasciola hepatica). 

 (Nearly twice natural size.) From Kellogg 

 and Doane. 



