216 TEXTBOOK OF ZOOLOGY 



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 simply swim along, 

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

 Instead of dying the injured worms 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, (8) 

 respiratory system (gills on parapodia of Nereis), and (9) improved 

 sense organs (eyes, palps, and tentacles of Nereis). 



