Medicinal Leech. 127 



39. Medicinal Leech (Hirudo Medicinalis), 



Showing the terminal suckers, the segmentation and annulation of the body, and 

 the distinctively coloured dorsal bands which difiFerentiate the variety Hirudo 

 medicinalis from the variety Hirudo officinalis. 



The number of annuli may be taken as about one hundred ; but 

 these annuli appear to be due merely to secondary con-ug-ation of 

 the primary seg-ments of the body, each of which comprises from 

 three to five of the secondary annuli. The primary segments are 

 not so readily distinguishable as the smaller rings ; but in a freshly 

 killed specimen, two white spots on either side the median line 

 and in a line with the central pair of eyes are considered to mark 

 out the anterior boundary of each segment ; whilst the posterior 

 boundary is given by the openings of the two muciparous or seg- 

 mental organs on the ventral surface, from which jets of fluid can 

 be made to issue by pressure, especially on the posterior part of 

 the body. The black pigment specks which are seen in this 

 variety upon the outer and middle of the three tawny stripes 

 on either side of the dorsum seem, by attaining a great size and 

 prominence on every fifth annulus, to point in the same direction 

 as those more constant land-marks just specified, and to mark out 

 in each case a segment consisting of five annuli. Fewer annuli 

 are interposed between each pair of the dorsally placed white specks 

 at either extremity of the body, than in the intervening space 

 occupied by the middle regions of the body. Upon the anterior 

 sucker a pair may be found upon almost every one of four half- 

 circles of which it is made up, whilst the posterior sucker appears 

 at certain periods of its development to be made up of no less than 

 seven segments, to which seven ganglia subsequently fused to- 

 gether correspond, just as in the rest of the body each pair of 

 white specks on the dorsal surface corresponds to the nerve ganglia 

 on the ventral. This aggregation of segments at either end of 

 the body corresponds firstly to the externally visible concentra- 

 tion of the animal functions of special sense, prehension of food, 

 and locomotion in the region of either sucker ; and secondly to a 

 concentration of nerve ganglia and an abortion of the reproduc- 

 tive and depuratory or muciparous organs which are vegetatively 



