PHYLUM ANNULATA 



205 



with what we have met with in the Cheetopoda, and is to be 

 looked upon as a mark of higher differentiation. 



The alimentary organs are greatly modified in accordance 

 with the blood-sucking habits of the animal. Surrounding 

 the mouth are three jaws, 

 one median and dorsal, the 

 other two ventro - lateral. 

 Each has the form of a com- 

 pressed muscular cushion, 

 with a sharp, evenly curved, 

 free edge covered with chi- 

 tin, which is produced into 

 numerous serrations or teeth 

 (Fig. 118). By means of its 

 muscles each jaw can be 

 moved backwards or for- 

 wards through a certain arc, 

 and the three, acting to- 

 gether, produce the characteristic triradiate bite in the skin 

 of the animal upon which the leech preys. 



The mouth leads into a muscular pharynx (Fig. 119, ph), 

 situated in the fourth to seventh segments. Radiating 

 muscles pass from its walls to the integument, and by their 

 contraction dilate its cavity and suck in blood made by the 

 jaws. Around the pharynx are numerous unicellular sali- 

 vary glands, which open close to the mouth ; their se- 

 cretion has the effect of preventing the coagulation of the 

 blood taken as food. 



The pharynx communicates by a very small aperture with 

 the second and largest division of the enteric canal, the 

 huge crop (cr), a thin- walled tube extending from the eighth 

 to the eighteenth segment, and produced into eleven pairs of 

 lateral pouches (cr, cr. 1, 11) . The crop is capable of great 



a 



Fig. 118. —a. Head of Hirudo medici- 

 nalis, showing the three jaws ( k) : b, 

 one of the jaws isolated, with the finely 

 toothed free edge. (After Sedgwick.) 



