494 ZOOLOGY 



SECT. X 



CLASS IV.- HIRUDINEA. 



1. EXAMPLE OF THE CLASS The Medicinal Leech (Hirudo medici- 

 nalis and H. (Limnobdella) austmlis). 



The medicinal Leech is found in ponds, swamps, and slowly 

 flowing streams in many parts of the world. H. medicinalis is the 

 common British species : H. australis is an allied Australian 

 form. 



External Characters. The Leech 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 

 proportions. It moves by " looping " movements, and is also a 

 good swimmer. The body (Fig. 411) is depressed or flattened 

 dorso-ventrally, the dorsal surface convex, the ventral flattened. 

 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 

 animal is brightly coloured, the dorsal surface in H. medicinalis 

 being longitudinally banded with alternate stripes of greenish- 

 grey and rusty red, the ventral surface greenish-yellow, spotted 

 with black : in H. australis the whole under-surface is rust- 

 coloured. 



The whole body is encircled by close-set transverse grooves, 

 dividing it into annuli. These, like the annuli of some Earth- 

 worms, 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. There are also 

 external characters by which the actual segmentation is plainly 

 indicated. The rust-coloured streaks on the back of H. medicinalis 

 are spotted with black, and at every fifth annulus the spots are 

 larger than on the intervening rings : the annuli thus marked are 

 the second of their respective segments. Moreover, the same 

 rings bear on the ventral surface minute paired apertures, the 

 nephridiopores or excretory apertures (np. 1, np. 17) : of these there 

 are altogether seventeen pairs, marking the first ring of the seventh 

 and the second ring of the eighth to the twenty-third segments. 



In front of the first and behind the last pair of nephridiopores 

 one important external mark of segmentation fails, but a further 

 indication is furnished by the presence on the middle ring of each 

 undoubted metamere of a number of delicate transparent elevations, 

 the segmental papillce (s.p.) } which have probably a sensory func- 

 tion. These structures are found along the whole length of the 

 body, and as they mark the middle (third) ring of all those segments, 



