250 



METAZOAN PHYLA 



286. Hirudinea. — The class Hirudinea contains the leeches, which 

 differ from other annelids in that they possess two suckers, one inclosing 

 the mouth and the other being ventral to the anal opening. They also 

 have a smaller number of metameres than other annelids. There appears 

 to be a greater number of these, however, than really exist, because each 

 one is marked by several transverse grooves (Fig. 157). 



Anterior 

 sucker 



Joiws 

 Mouth 



Opening of 

 vois deferens 



Opening of 

 ovielucr 



Nephrielio 

 pore 



Sense 

 poipilloie 



Mouth 

 Pharynx 



fat diverticulum 

 of crop 



lOth diverticulum 

 of crop 



Stomach 



Intestine 



■ Rectum 



-Anus 



Posterior 

 sticker ' 



Fig. 157. — The medicinal leech, Hirudo medicinalis Linnaeus. {From Borradaile, 



"Manual of Elementary Zoology," by the courtesy of Oxford University Press.) A, the whole 



animal, ventral surface. The segments are shown by Roman numerals. B, the anterior 



end, dorsal surface. C, the posterior end, dorsal surface. D, the digestive system. A, 



natural size; B and C slightly enlarged. 



The suckers are used as organs of attachment when the animal is at 

 rest and also as organs of locomotion when the animal moves about upon 

 a firm substratum, the movement being similar to that of a measuring 

 worm. Certain leeches are able to swim freely through the water, the 

 body performing vertical undulations. 



The mouths of some leeches, such as the medicinal leech (Fig. 157), 

 are provided with jaws armed with chitinous teeth. When such a leech 

 attaches itself to another animal for the purpose of securing blood these 



