SECT, ix PHYLUM ANNULATA 205 



end bears a disc-like posterior sucker (p. j.), also directed 

 downwards, and at its junction with the trunk, on the dorsal 

 surface, is the very small median amis (a.). 



The whole body is encircled by close-set transverse 

 grooves, dividing it into annuli. These, like the annuli of 

 some earthworms, are more numerous than the true 

 segments or metameres, the study of the internal organs 

 showing that, except at the two extremities, each segment con- ' 

 tains five annuli. On the ventral surface of the fifth annulus 

 of each segment is a pair of minute apertures, the nephridio- 

 pores or excretory apertures (np. 1-17) : of these there are 

 altogether seventeen pairs, marking the fifth annuli of the 

 sixth to the twenty-second segments. 



The anterior sucker bears on its dorsal surface five pairs 

 of small black spots, the eyes (e. i, e. 5). 



The perfectly definite and comparatively small number of 

 metameres in the leech offers a striking point of contrast 

 with what we have met with in the Chaetopoda, 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 compressed 

 muscular cushion, with a sharp, evenly curved, free edge 

 covered with chitin, and is produced into numerous serra- 

 tions or teeth : by means of its muscles each jaw can be 

 moved backwards or forwards through a certain arc, and the 

 three, acting together, produce the characteristic triradiate 

 bite in the skin of the animal upon which the leech preys. 



The mouth leads into a muscular pharynx (Fig. 112, 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 from the 



