39 



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. The leech lives on the 

 blood of higher animals : it lays its eggs in a cocoon, which 

 it buries in a hole in the bank of the pond it inhabits. 



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 bod^. The segments or somites, 

 as indicated by the internal organs, are much less numerous 

 than the annuli, five of these latter corresponding to each 

 somite, except near the ends of the body. 



This segmental arrangement affects markedly the nervous, 

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

 circulatory and digestive organs. 



Leeches intended for dissection should be killed with 

 chloroform. 



I. EXTERNAL CHARACTEES. 



1. The shape varies greatly with tlie degree of elongation 

 or contraction. The body is broadest a little way 



