450 BIOLOGY AND HUMAN LIFE 



330. Worms. From a practical point of view worms are important 

 because each of the groups contains members that are parasitic upon 

 man or upon domestic animals (see Chapter XXXII). Some of the 

 parasites of the flatworm group, living upon oysters and other mollusks, 

 bring about the formation of pearls. For our own protection from 

 many of these parasitic w^orms our foods must be properly cooked, and 

 public agencies must prevent the passing of infected meat through mar- 

 kets or slaughterhouses. 



331. Echinoderms. The one of most practical importance in our 

 country is the starfish, w^hich preys upon the oyster and has thus caused 

 considerable damage to the oyster beds. In the Orient the trepang, or 

 sea cucumber, is gathered for food. 



332. Annelids. The segmented worms, of which the common earth* 

 worm is typical, are often used as bait by fishermen. The earthworm 

 bores into the ground by swallowing bits of earth, which pass through 

 the body and are thrown out as castings. In this way two results are pro- 

 duced : (i) the earth is brought from the lower layers to the sur- 

 face, and (2) the soil is made more porous, allowing freer movement of 

 water and air. In addition the earthworm consumes large quantities of 

 organic matter in the soil, and some of the decomposition products are 

 combined with the earth passing through the gut. It thus contributes to 

 the fertility and to the mechanical qualities of the soil. 



333. Arthropods. This is a very large branch of the animal 

 tree, with its divisions fairly v^ell marked. There are four 

 important classes. 



1. Class Myriapoda (thousand legs). The common house 

 centipede is useful in that it kills small insects. It is perfectly 

 harmless to man. Some other centipedes destroy earthw^orms and 

 small snails, as well as insects, and their sting is very painful. 



2. Class Crustacea. This class is represented by the lobsters, 

 crayfish, crabs, and shrimps, which are useful as food ; by the 

 barnacle, which becomes a nuisance on the bottoms of vessels ; 

 and by the sow bug. Aside from the food which they yield, 

 these animals are important factors in the lije cycle, since some 

 of them are almost microscopic in size, while some of the lob- 

 sters grow to be two feet or more in length. The crayfish some- 

 times cause injury to vegetation and to earth dams. Some of 

 the crabs have become parasitic on fishes. 



