EARTHWORM 357 



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. 



The medicinal leech was once used in the bleeding of individuals 

 as a treatment for various ailments. Various forms of leeches live 

 as parasites on turtles and other forms of animal life in the water. 

 They are not at all averse to attacking human beings when the 

 opportunity presents itself; however, they cause no great injuries 

 and are important only as pests and as secondary hosts for some 

 parasites, thus spreading certain diseases. 



Marine annelids are important only as food for larger forms. In 

 many regions the burrowing forms along the tide levels literally 

 form a good grazing ground for fishes. The fish swim along, biting 

 off that part of the worms protruding from the mud or sand. In- 

 stead of dying, the injured worms simply regenerate one or more 

 new heads and go about their business. 



Phylogenetic Advances of Annelida 



(1) Segmentation, (2) coelom, (3) alimentary canal with defined 

 parts, (4) closed circulatory system, (5) excretory system of 

 nephridia, (6) muscular system, (7) concentrated mid-ventral nerve 

 cord connected to a dorsal pair of suprapharyngeal ganglia. 



References 



Beddard, F. E.: Earthworms and Their Allies, London, 1901, Cambridge Uni- 

 versity Press. 



Borradaille, L. A., and Potts, F. A.: The Invertebrata, New York, 1932, The 

 Macmillan Company. 



Hegner, E. W. : Invertebrate Zoology, New York, 1933, The MacMillan Company. 



Parker, T. J., and Haswell, Wm. A.: Textbook of Zoology, Vol. I, New York, 

 1930, The Macmillan Company. 



Olson, Henry W. : The Earthworms' of Ohio, Ohio Biol. Survey Bull. 17, Vol. IV, 

 No. 2. 



