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Chapter IV. 

 THE LEECH. Hirudo medicinalis. 



The leech is an elongated flattened worm, from three to five 

 inches in length, and provided with a muscular sucker at 

 each end. The body is marked externally by a series of trans- 

 verse constrictions dividing it into rings or annuli ; and is 

 capable of considerable elongation and contraction. 



Leeches occur in freshwater pools and marshes in this 

 country, but far more abundantly on the Continent. They 

 either swim freely by vertical undulations of the body, or 

 progress in a looping manner, attaching themselves alternately 

 by the anterior and posterior suckers. They live on the blood 

 of higher animals ; and they lay their eggs in cocoons, which 

 are buried in holes in the banks of the ponds they inhabit. 



Leeches are ' segmented animals,' i.e. several of the organs 

 are repeated, usually in pairs, at regular intervals along part or 

 the whole of the length of the body. The segments or somites, 

 as indicated by the internal organs, are much less numerous 

 than the annuli, of which five correspond to each somite, 

 except at the ends of the body. 



This segmental arrangement affects in a marked manner 

 the nervous, excretory, and reproductive systems, and to a less 

 degree the circulatory and digestive organs. It appears to 

 result feom a definite arrangement of parts which, in the ances- 

 tors of leeches, were scattered irregularly through the body, 

 much as in Fasciola. 



Leeches may be readily killed either with chloroform, or 

 by plunging them into a hot solution of corrosive sublimate. 



