PHYLUM ANNELIDA 



237 



differ externally from the flatworms (Platyhelminthes, Chap. 

 VII) in being distinctly segmented. The external segmenta- 

 tion, however, does not correspond exactly to the internal seg- 

 mentation, since there are a variable number of external grooves 

 (from two to fourteen) to every real 

 segment, e.g. usually five in the me- 

 dicinal leech, Hirudo (Fig. 169), and 

 its allies, and three in Clepsine. 

 Anatomical features which distinguish 

 the Hirudinea from the Archian- 

 nelida and Ch^etopoda are (1) the 

 presence of a definite number of seg- 

 ments (thirty-three), (2) two suckers 

 (Fig. 169, 1, 2), one formed around 

 the mouth and the other at the pos- 

 terior end, and (3) the absence of 

 setae (except in one genus). They 

 are hermaphrodites. 



Hirudo medicinalis, the medicinal 

 leech (Fig. 169), is usually selected as 

 an example of the class. It is about 

 four inches long, but is capable of 

 great contractions and elongation. 

 The suckers are used as organs of 

 attachment, and during locomotion 

 are alternately fastened to and re- 

 leased from the substratum, the animal FlG - 169. - - A leech, Hirudo 



. . . . . medicinalis. i, mouth; 2, pos- 



looping along like a measuring- worm, terior sucker; 3, sensory papil- 



Leeches are also able to swim through lae - ( From Shipley and Mac- 



,, , , . 6 Bride.) 



the water by undulating movements. 



The alimentary tract (Fig. 170, i-y) is fitted for the digestion 

 of the blood of vertebrates, which forms the principal food of 

 some leeches. The mouth lies in the anterior sucker (Fig. 169, 1) 

 and is provided with three jaws armed with chitinous teeth for 

 biting. The blood flow caused by the bite of a leech is difficult 



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