castle: north AMERICAN RHYNCHOBDELLID/E. 31 



Their ventral capsules show the following modification in arrangement ; 

 they have been displaced from the typical tandem position to a side-by-side 

 position (Figures 8, 10; compare Figure 9, somites xxix.-xxxiii.). 



The lateral capsules of ganglion No. 2 are found dorsal to the pharyngeal sac 

 (2, '2, Figures 8, 12). They seem to have been displaced backward to a po- 

 sition somewhat posterior to the lateral capsules of ganglion No. 3 by a migration 

 in that direction of the supra-oesophageal commissure (Figure 8 ; compare 

 Fi'^ures 11, 21). The commissure in this species is normally thrust back of 

 the position in which it is shown in Figure 8, so that it lies about over the 

 lateral capsules of ganglion No. 5. The animal whose brain is represented in 

 Figure 8 was curved ventrad so that the commissure was thrust forward of its 

 usual position and the row of lateral capsules below it was straightened out 

 a little. The position of the ventral capsules of ganglion No. 2 is shown in 

 Figures 8 and 10 ; the nerve root (il., Figure 8) arises at the anterior end of 

 the brain just ventral to nerve root i. 



The ganglionic capsules of neuromere No. 1 all lie dorsal to the pharyngeal 

 sac and anterior to the supra-oesophageal commissure (Figures 8, 12). I be- 

 lieve that the most anterior and ventral of these (ly., Figures 8, 12), which 

 lies closely attached to nerve root l. in each half of the body, is homologous 

 with a ventral capsule of one of the succeeding ganglia. Capsule 1 1'. extends 

 out lateral to, sometimes even ventral to, nerve root I., so that its end niay 

 appear in sections between nerve roots i. and il. 



Oka ('94) states that he finds in the brain of " Clepsine " (Glossiphonia) 

 always thirty nerve capsules, and he accordingly regards it as equivalent to 

 five fused ganglia and no more. Since G. stagnalis was one of 'the species 

 studied by him, I am unable to understand how he can have reached such a 

 conclusion, unless he has overlooked altogether the capsules of somite I. which 

 lie anterior to the supra-oesophageal commissure. 



Both the number and arrangement of tlie nerve capsules, and the number and \ 

 position of the nerve roots, show clearly that m G. stagnalis Six fused ganglia are) 

 represented m the brain, and that in the entire body thirty-four somites ara 

 represented. ' 



(2) Somite Limits. 



It remains to explain the grounds on which the limits of the somites have 

 been placed by me as indicated in Figure 4. Wliitman ('85) pointed out 

 many years ago that a certain ring (the first, according to his account) of each 

 typical somite in the body of a leech is more richly supplied with sensory 

 organs (" sensilloe ") than any other ring of the somite. In many species of 

 Glossiphonia special color markings or papillre are also found on the sensory 

 ring. Color markings, however, are wanting in Q. stagnalis, and the sen- 

 sillie are not sutficiently conspicuous in tiie living animal to make identification 

 of the sensory rings at all certain. But a carmine stain of the proper intensity 

 renders identification of the sensory rings quite easy by giving them, especially 

 along the margins of the body, a son)cwhat darker color. Observing this fact,/ 

 VOL. XXXVI. — NO, 2. 2 



