82 PROCEEDINGS OF THE AMERICAN ACADEMY 



Every Ilirudo has 26 somites, counting from the first pair of eyes to 

 the acetabulum: 10 of these — the first G and the l<(st A — are abbre- 

 viated by the S2ippression of from 2 to 4 rings in each ; and IG, lying 

 between the frst (\stp) and the last pair of ni'phridlal pores (\ltli p), 

 have each five rings. Tlie G anterior somites include 13 rings, the 

 first and second being represented each by a single ring, the third by 2 

 rings, and the fourth, fifth, and sixth, each by o rings. Tlie 4: posterior 

 somites embrace 9 rings (94-102), the twenty-third somite including 3 

 rings, and the twenty fourth, tiventy fifth, and twenty-sixth, each 2 rings. 



The first ring of each somite is marked anteriorly, by a pair of eyes ; 

 and, from the Wth. ring onicard, by the segmental jiapilla;, of which 

 there are normally from G to 8 on the dorsal hcdf of the ring, and G on 

 the ventral half. 



The first pair of eyes replaces a pair of median papillcc ; and the 

 remaining four pairs of eyes replace as many pairs of the inner lateral 

 papillce {il}- 



The eye-bearing rings arc the \st, 2d, Sd, bth, and Sth. 



The buccals are the 5th and Gth, which are united on the ventral side. 

 The post-buccals are the 1th and 8ih, also united ventraUy. 



There are seventeen pairs of nephridial pores located in the hind 

 edge of the ventral half of the following rings: — \otIi, 18th, 23d, 28th, 

 33d, 38th, 43d, 48th, b3d, o8th, G3d, GSlh, 73d, 78ih, 83d, 88th, and 

 03d. Each pair is thus in the last ring of its somite. Between the 

 first {\st J)) a)id the last (\.lth p) pair there are precisely IG complete 

 somites (7-22 inclusive), or 80 rings (14-93 inclusive). 



The male orifice lies between the 30th and 31 st rings, or the 2d and 

 3d rings of the lOth somite. The female orifice is 5 rings behind the 

 male, between the 3i)th and 3(jth rings, or the 2d and 3d rings of the 

 Wth somite. 



The clitellum inchides the 9th, 10th, and 11th somites. 



Tlie amis lies in the 102d ring, or between this and the preceding one. 



The other characters of this genus are ba.-^ed on internal organs, 

 and are too well known to require repetition here. 



We may now consider the abbreviated somites, and see what they 

 have to tell us about the history and relationship of the ten-eyed 

 leeches. We notice first of all that the abbreviation is greatest at 

 the extreme ends, from which it is plain that it began at these points 

 and progressed centripetally ; i. e. towards the middle of the body. 

 The first two somites have lost each 4 rings ; the third, 3 rings ; the 

 fourth, fifth, and sixth, each 2 rings; making a total loss of 17 

 rings at this end. The twenty-third somite has lost 2 rings, and the 



