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A.utolytus . A chain of individuals of one sex is developed. 

 The hindmost (the first-formed) being the 

 original sexual half of the " heterosyllid " ; 

 the intervening ones are produced by bud- 

 ding from the zone of fission. All acquire 

 heads before separation. 



Myrianida . A chain of individuals of one sex is developed 

 by budding from the tail of an asexual 

 worm, the hindmost zooid being the one 

 first formed. 



yllis ramosa. This worm forms lateral buds, and these in 

 turn form buds, the result being a much- 

 branched colony. Some of the individuals 

 develop sexual organs and go free. 



CLASS HIRUDINEA, OR DISCOPHORA 

 (LEECHES). 



Hirudo medicinalis (The Medicinal Leech). 



Describe the External Features of the Leech. 



The flattened body, which is regularly marked on the 

 <dorsal surface and mottled on the ventral surface, has, 

 at the front end, a small anterior sucker within which is 

 the mouth, and, at the terminal end, a larger blind pos- 

 terior sucker. The anus is dorsal, in the constriction in 

 front of the posterior sucker. There are about 100 skin 

 rings on the body. Five rings correspond to a segment, 

 -except at the ends of the body ; and most of the segments 

 are indicated by the large dark spots on every fifth ring. 

 On the dorsal surface at the anterior end there are five 

 pairs of " eyes." On the ventral aspect, between rings 

 30 and 31, is the male aperture (from which the penis is 

 sometimes seen protruding) ; and five rings behind jt is 

 the female aperture. Also ventral are the openings of 

 the nephridia (seventeen pairs), each pair five rings apart. 



How does the Leech move ? 



In a looping way, by alternately fixing and releasing 

 its suckers. 



