MEDICINAL LEECH. 213 



first pair of eyes replaces the median pair of papillae, the remaining four 

 pairs a papilla of the inner series. The first and second annuli of the 

 body carry a pair of eyes ; the third annulus does the same, but it is fol- 

 lowed by an annulus with neither eyes nor segmental papillae : and the 

 two therefore go together. The fifth annulus bears eyes ; the eighth and 

 eleventh only segmental papillae, and these three annuli (fifth, eighth, and 

 eleventh) are each followed by two annuli which have neither eyes nor 

 papillae. Whitman therefore concludes that the first and second somites 

 are represented by a single annulus, the third by two annuli, the fourth 

 to the sixth inclusive by three apiece. The seventh and succeeding somites, 

 up to the twenty-second inclusive, are each composed of five annuli, the 

 first of them bearing the segmental papillae. The twenty-third somite has 

 three, the twenty-fourth to the twenty-sixth two annuli apiece. 



The buccal annuli are the fifth and sixth, the post-buccals the seventh 

 and eighth, and these two pairs of annuli are fused ventrally. A pair of 

 nephridial pores opens on the ventral surface of the last annulus of the sixth 

 to the twenty-second somite inclusive. This annulus can be readily recog- 

 nised in H. medicinalis by the fact that it is the one that carries a large 

 black spot in the middle of the three light lines which traverse the body 

 lengthwise on each side of its dorsal aspect. The male orifice lies between 

 the second and third rings of the tenth somite, i. e. the thirtieth and thirty- 

 first annuli of the body. The female orifice occupies a corresponding 

 position in the eleventh somite. The ninth, tenth, and eleventh somites 

 constitute the clitellum or region which secretes the cocoon. The anus lies 

 either in the last annulus or in front of it. It may be added that papillae 

 occur also on the sucker, but they do not afford any clue to its composition. 



The grouping of the annuli, as above detailed, is characteristic of the 

 genus Hirudo. Slight differences are observable in allied genera. 



The medicinal Leech varies much in its colouration : and no less than 

 sixty-four varieties have been enumerated. The variety H. medicinalis has 

 in the natural state the dorsal surface greenish grey, with three rust-red 

 longitudinal streaks on either side. The middle one of these three streaks 

 has a black spot more or less distinct on each annulus, and it may be readily 

 seen that one of these spots at regular intervals is much enlarged. It marks 

 the last annulus of a somite. There is also a small black spot in the same 

 annulus interrupting the inner light line. The ventral surface is greenish 

 yellow spotted with black, or else black. The action of the spirit soon de- 

 stroys these bright colours, as it has done here. H. officinalis has a median 

 dorsal green band bordered by a red or brown line. The lateral dorsal 

 regions are green with black and reddish-brown spots, sometimes grouped 

 in two longitudinal lines. The amount of black pigment is very variable, 

 and sometimes the red prevails. The ventral aspect is green, and as a 

 rule not spotted. There are many intermediate forms between these two 



