PBO< l l DING9 "l l HE AMERICAN k( \M.MY. 



him their limits. These were, first, Borne whitish fleckc llae) found 



dorsallj on the ring which contains the ganglion (sensory rin{ condly, 



a pair of pores (nephropores) lying in the furrow jusl posterior to ring 5, 

 counting the Bensory ring as ring 1. 



Gratiolel Beems to have assumed thai these convenient landmarks 

 indicated respectively the anterior and posterior limits of the mor- 

 phological units or Bomites. In this assumption nearly all subsequent 

 investigators have followed him unquestioningly. Vaillanl (*70) alone 

 expressed the view that it would be more logical to look for -■ pta, which 

 Bhould, as in the Chaetopoda, mark tlie limits of the somites. In the 

 case of Pontobdella, Vaillant believed that he had succeeded in finding 

 Mich septa, but subsequent investigators do not agree with him in this 

 \ iew. 



Gratiolet further observed that toward the ends of the body the 

 number of rings in a somite becomes smaller, and the ganglia are 

 crowded closer together. He suggested that the terminal ganglion at 

 either end of the body probably had resulted from a fusion of originally 

 distinct ganglia, as is the case with the ganglia of certain Mollusca 

 i Helix, etc.). 



Gratiolet thus opened the way for a study of the metamerism of the 

 leech in two directions: first, to determine the number of primitive 

 somites present in the body of the leech; and secondly, to determine the 

 natural limits of these somites. The first problem indicated has received 

 B satisfactory solution through the splendid work of Whitman f92) on 

 the Rhynchobdellidffl, and of Bristol ('99) on the Gnathobdellidse. 



From studies of my own, the results of which are in process of pub- 

 lication, I can confirm the conclusion of these investigators, that the 

 body of the leech contains thirty-four distinct somites. 



The second problem, namely, the determination of the true limits of 

 the somite, has M already indicated, received practically no attention. 

 To a discussion of this problem and some considerations growing out of 

 it. I now invite attention. 



II. Limits of the Somite in the Rhtnchobdellid^. 



1. Neuromere* "•■>• " Criterion of Somite Li mils. 



In determining the number of somites found in the body of the leech, 



it has been found necessary to rely solely on a stud\ of the nervous 

 Bystem. It would seem natural, therefore, to look in the same direction 

 tor a solution of the problem concerning the limits of the somite. 



