32 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
I was first enabled to determine as sensory the rings indicated by Arabic 
numerals in the right half of Figure 4 ; further study revealed the presence of , 
marginal sensille in the positions indicated in Figure 3. 
The metamerically repeated sensory annuli were thus positively identified 
throughout the greater part of the body. It remained merely to mark off the 
somite limits between successive sensory annuli. This I at first did after the 
usage of Whitman (’85, 92) and practically all others since the time of Gra- 
tiolet (62), considering the sensory ring as occurring at the anterior end of 
ats somite. 
I found, however, that a consistent following of this practice would, toward 
either end of the body, place the somite limits in the middle of a ring instead 
of between rings, the position in which somite boundaries fall in other regions 
of the body. See Figure A, xXv’., xxvr’,, etc. 
XXVIL? <8. 
Fieure A.— G. stagnalis. Dorsal view of posterior part of body, showing mar- 
ginal sensilla. Somite limits are indicated correctly at the right of the figure 
(xxIv. to xxvit.); at the left of the figure (xx111’. to xxvm’.) they are shown 
as they have been commonly but incorrectly placed. 
This led me to inquire whether the sensory ring really is the anterior ring 
of its somite. The results of this inquiry have been published elsewhere 
(Castle, 1900), so that only one or two of the more important conclusions 
need be restated here. One of these, already suggested in part on page 29, 
is the following : — 
Somite limits coincide with neuromeric limits ; consequently in Glossiphonia the 
sensory ring is the middle, not the anterior ring of the somite. 
