170 



PKACTICAL PAKASITOLOGY 



swellings, placed radially, will be seen (fig. 87, a) ; these contain the 

 jaws. 



The body covering shows circular furrows placed very close 

 together, which divide the surface into a number of narrow annula- 

 tions. In H. medicinalis there are 102 annulations ; but, with the 

 exception of the anterior two and the one which bears the anus, they 

 do not correspond to any segmentation of the body- 

 cavity. In the larger portion of the body, consisting 

 of 17 segments, 5 annulations go to make up a 

 body segment, while the anterior segments are com- 

 posed of 3, the posterior of 2 (or 1) annulations re- 

 spectively. It requires very minute examination to 

 make out the margins of the body segments from the 

 exterior. Indications are offered by the sensory 

 organs of the skin, 1 which are placed, 6 to 8 dorsally 

 and 6 ventrally, upon the first annulation of each 

 segment ; and by the orifices of the 17 pairs of 

 excretory organs (nephridia, segmental organs, 

 looped ducts), placed at the sides of the ventral sur- 

 face upon the last annulation of each segment, and 

 only to be seen with the glass. The sexual open- 

 ings are more easily found. They are placed upon 

 the ventral surface in the median line ; the male 

 SB orifice, from which the white thread-like penis fre- 

 quently depends, being situated in the 10th body 

 segment, between the 30th and 31st annulations ; 

 while the female orifice is behind it, in the llth 

 segment, between the 35th and 36th annulations. 



In all Hirudinea the width of the body-cavity 

 is reduced by longitudinal canals, resembling blood- 

 vessels in appearance, which run the entire length 

 of the body and are not anatomically distinct from 

 the body-wall. The presence of these canals renders 

 the task of exposing the organs somewhat difficult. 

 The leech should be opened upon the dorsal aspect, 

 care being taken not to penetrate the intestine. A 

 good deal of practice is required to perform this 

 manipulation successfully. 



The best method is to lay the parasite upon its 

 ventral surface and, after stretching, to secure it by 



FIG. 86. - Show- 

 ing the anatomical 

 structure of Hiru- 

 dinea. The dorsal 

 surface has been re- 

 moved and a portion 

 of the intestine, be- 

 tween the penis and 

 the first pair of 

 testes, is cut away, 

 showing the female 

 sexual organs. The 

 loops at the lateral 

 margins represent 

 the nephridia. (After 

 Kennel.) 



1 Upon the 1st, 2nd, 3rd, 5th, and 8th annulations, two of the dorsal organs are, in 

 each case, replaced by eye-marks. When seen with the magnifying glass, these 

 appear as black spots with a regularly defined margin. 



