CASTLE: ^■ORTH AMERICAN RHYNCIlOBDELLIDiE. 29 



ganglionic mass which lies in the posterior sucker and supplies it with nerves ; 

 these, added to the twenty-one distinct ganglia found in the central part of the 

 body, bring the total up to thirty-four. An examination of the sense organs 

 connected with these ganglia, and situated typically on the middle ring (hrst, 

 Whitman) of each somite, yields corroborative evidence that the number of 

 somites represented in the body is thirty-four. 



Bristol ('99) subsequently made a similar study of the metamerism of 

 Nephelis lateralis, his conclusions being for the Gnathobdellidse entirely in 

 harmony with those of Whitman for the Rhynchobdellidte. 



Oka ('94), however, has cast doubt upon the general applicability of Whit- 

 man's determination, based as it was on the metamerism of a single species of 

 Glossiphonia, by stating that in the several European species which he has 

 studied (Q. stagnalis, G. complanata, G. concolor, G. heteroclita, G. papillosa, 

 G. marginata, and G. tessellata) he finds evidence of only Jive (not of six) fused 

 ganglia in the brain. Moreover, in recent systematic papers, such as those of 

 Blanchard ('94) and Moore ('99), we find the body of the leech still analyzed 

 and described as consisting of twenty-six preanal somites, instead of twenty- 

 seven, the number found in that portion of the body by Whitman ("92) 

 and Bristol ('99), and still earlier, though on less satisfactory evidence, by 

 Apathy ('88). 



Accordingly, I have thought it worth while to examine into this matter 

 rather carefully in the case of the species studied by me. 



I may say at once that my results, in the case of all six species studied, are 

 in complete accord with those of Whitman ('92), so far as the number of meta- 

 meres is concerned. In determining the limits of the somite, I have arrived at 

 conclusions differing from those of my predecessors, as will presently appear 

 (p. 31 ff.). 



a. Structure of a Typical Ganglion. — A typical ganglion from the middle 

 of the body has its ganglion cells arranged in six groups enclosed in capsules of 

 connective tissue. Four of these capsules are lateral in position, two on each 

 side of the ganglion ; the other two occupy a mid ventral position, one in the 

 anterior, the other in the posterior part of the ganglion. (See the ganglion of 

 somite xxvi. in Figure 9, Plate 3.) Three nerves are given off close together 

 from either side of the ganglion, and are distributed to the three successive rings 

 of one and the same somite, as I have elsewhere (Castle, 1900) pointed out. 



If, then, we can determine exactly how many such ganglia are present in the 

 oentral nervous system of a leech, we shall be in a position to say how many 

 somites enter into the composition of its body. 



In the middle part of the body, as already stated, twenty-one distinct ganglia 

 of the sort just described can easily be recognized. To determine how manv 

 are present toward either end of the body, where more or less fusion of gant^lia 

 has taken place, is a matter of more difticulty. 



^. Fused Ganglia. — Figure 9 (Plate 3) shows a dorsal view of the poste- 

 rior part of the central nervous system of G. sUxgnalis, obtained by reconstruc- 

 tion from a series of frontal sections. The last two distinct ganglia, those of 



