PHYLUM ANNELIDA 



215 



male has no proboscis, is ciliated, and lives in the segmental organ of 

 the female. Representatives of the order Sipunculoidea have no 

 prostomium in the adult. 



Nephrostomo 



Male genital pore 



Alimentary canal 



Alimentary canal 



Anus 



Fig. 114. — Bonellia viridis. Female (above) has bifurcated proboscis ; male 

 (below) is ciliated over the surface and much smaller. (From Hegner, College 

 Zoology, published by The Macmillan Company.) 



Importance of Annelids to Man and Other Animals 



Even though no casual observer would consider that annelids 

 have any important relationship to other living organisms, they 

 have been found to be of great importance in a number of ways. 

 Darwin concluded from some forty years of observation that the 

 earthworms in an acre of ground could bring to the surface in one 

 year as many as eighteen tons of feces, known as castings. This in- 

 dicates without doubt that these animals are of great value, because 

 in stirring the soil they cover up objects, causing them to decay. 

 Their continuous burrowing through the soil also makes it porous, 

 a necessary condition for plant growth. 



Earthworms have also less desirable qualities. They serve as 

 secondary hosts for parasites of several animals. Most of the para- 

 sites having the earthworm as a secondary host live as adults in 

 birds, pigs, and other animals which use the worms as food. 



They have created a serious problem in some of the irrigation 

 districts of the Southwest by burrowing through levees until they 

 are too porous to hold water. Before irrigation was started they 

 did not appear to be at all numerous, but with the presence of water 

 they have become very abundant. 



