PHYLUM ANNULATA 203 



2. THE HIRUDINEA 



A good example of the Hirudinea is the medicinal leech 

 (Hirudo), various species of which are to be found in ponds, 

 swamps, and slowly flowing streams in various parts of the 

 world. 



It is a vermiform animal, some 6-10 cm. (2-3 inches) in 

 length, but is capable of contracting and elongating itself 

 so as to produce great alterations in form and proportion. 

 It moves by " looping " movements, and is also a good 

 swimmer. The body (Fig. 117) is depressed or flattened 

 dorso-ventrally, the dorsal surface convex, the ventral flat- 

 tened. The anterior end presents a ventrally directed, cup- 

 like hollow, the anterior sucker (a. s) , in the middle of which 

 is a small aperture, the mouth (mth). The hinder end bears 

 a disc-like posterior sucker (p.s), also directed downwards, 

 and at its junction with the trunk, on the dorsal surface, is 

 the very small median anus (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 contains five 

 annuli. On the ventral surface of the fifth annulus of each 

 segment is a pair of minute apertures, the nephridiopores 

 or excretory apertures («. p. i-ij ) ; of these there are 

 altogether seventeen pairs, marking the fifth rings 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. 1, e. 5). 



The perfectly definite and comparatively small number of 

 metameres in the leech offers a striking point of contrast 



