296 PROCEEDINGS OF THE AMERICAN A< A DEMY. 



from the sensory ring, '■'>. In Bomite XXVII. a complete onion has 

 taken place between rings 1—3; rings l and 5 are probably repre- 

 ted in the broad, curved pari of the body which bears distally the 

 acetabulum. This pari is imperfectly divided by transverse erea 

 (shown too distinctly in the 6gure), but is Bharply marked off from the 

 preceding ring. 



To recapitulate : — We find in the case <>f Nephelis the Bame rul< a gov- 

 erning the process of somite reduction as in the case of Glossiphonia. 



(1). Rings fuse into groups innervated typically from a common 

 ganglion. Examples: Figure 5, right half, Bomites XXV. XX VI I. ; 

 I jure 4, somites III. and IV. The union, at the margins of the body, 

 of somite I. with the anterior ring of somite II. presents an apparent 

 exception, hut here we have to do with an extreme case, in which "// the 

 rings of one somite (I.) have fused with one another, and in addition a 

 fusion between successive Bomites (I. and II.) i- foreshadowed. 



(2). Tltr first rings />> disappear are those at tin- ends of the somite, 

 the anterior end being usually the one which is first affected. Examples: 

 in Figure 4, somite VII. (only the most anterior ring, 1. wanting) ; so- 

 mite VI. (a ring wanting at either end of the somite) ; somite V. (two 

 rings wanting at the anterior end of the somite, only one wanting at 

 the posterior end); in Figure 5, somite XXVI. (rings 1 and 2 fused 

 together and partially muted with ring 3, rings 4 and 5 muted); somite 

 XXVII. (rings 1 ■"> united, rings 4 and ."> united). 



\n somite XXV. alone we have a ease where abbreviation affects the 

 posterior part of the somite sooner than the anterior part. But it will 

 he observed that the middle of somite XXV. marks the boundary 

 hetween the abbreviated and unabbreviated portions of the posterior 

 half of the body. This may explain why in this single instance rings I 

 and 5 are reduced, while rings 1 and 2 are unaffected. 



/■'mm //,/• foregoing facts it appears that in the case of Nephelis as well 

 us of Glossiphonia there are natural somite limits, which coincide with 

 the limits of the neuromeres. 



IV. Somite Growth. 



Having now examined with some care the process of somite reduction 

 in the leech, it may he instructive to study also the reverse proc< --. 

 namelj that of increase in the number of rings in a Bomite. This can 

 best be done by an examination of typical somites from various genera 

 of the two commonh recognized families of leeches. 



