CASTLE. — METAMERISM OP THE HIRUDINEA. 289 



somites I. and XXVII. a fusion has taken place between the sensory 

 ring and both the adjacent rings, the one which follows as well as the 

 one which precedes it. An intermediate condition is found in somites 

 II., XXV., and XXVI., where the ring following the sensory ring is 



XXIII.' 



xxiv/ s. y ^y J> xm 



xxv/ < . _ . xxv _ 



XXVII. 



Fig. 2. Posterior end of the body of Glossiphonia stagnate L. (Clepsine biocu- 

 lata Saw), dorsal view. Somite limits as commonly placed arc sliown at the left 

 of the figure (XXIII. '-XXVII. ') ; as the writer would place them, at the right 

 (XXIV.-XXVII.). 



considerably reduced in size, but still distinct from the sensory ring 

 anterior to it. The amount of shortening which the somite undergoes 

 is thus seen to be increasingly greater as one progresses from the middle 

 toward either end of the body. The successive steps of abbreviation 

 are : — 



1. A fusion takes place between the sensory ring and the ring which 

 precedes it. 



2. The ring which follows the sensory ring is reduced in size. 



3. It fuses with the sensory ring preceding it. 



Accordingly, at the conclusion of this process, we find united in a single 

 ring three primitively distinct rings, — a sensory ring and the two rings 

 adjacent to it. But these three rings are typically innervated from the 

 same ganglion (Fig. 1, VII., VIII.). This fact substantiates the view 

 already expressed, that neuromere limits coincide with somite limits. 



If we do not admit that neuromere limits coincide with somite limits, 

 vol. xxxv. — 19 



