CASTLE. — METAMERISM OP THE HIRUDINEA. 293 



Fig. 4. The first seven somites and part of the eighth somite of Nephelis 

 lateralis, dorsal view, a., anterior nerve ; d., dorsal branch of posterior nerve, 

 which supplies nervous structures in all five rings of a somite ; ami. n., nerve ring 

 found typically in each of the two annuli adjacent to a sensory annulus ; p, pos- 

 terior nerve ; z, a branch of the anterior nerve which ramifies in annulus 1 of 

 its somite. 



Roman numerals indicate the somite limits, at the left of the figure (I.'-VII/), 

 according to the view of Bristol; at the right of the figure (I.-VIII.), accordiug to 

 the writer's view. After Bristol ('99), simplified. 



phonia (Fig. 1) retains its separateness in Nephelis, and is distributed 

 chiefly to the two rings which follow the sensory ring. In Glossiphonia 

 the posterior nerve gives off a branch (d, Fig. 1), which is connected 

 with sense organs on the dorsal surface of all three rings. In Nephelis 

 the posterior nerve gives rise, to a homologous branch (d, Fig. 4), which 

 is distributed to sense organs on the dorsal surface of rings 1, 3, and 5, 

 and is also connected with the nerve rings (ann. n., Fig. 4) discovered 

 by Bristol iu rings 2 and 4< 



Hence, if neurotuere limits coincide with somite limits, the latter must 

 fall, in the case of somites VI.-VIII. of Nephelis, as indicated in the 

 right half of Figure 4.* Bristol, working on the basis of Gratiolet's 

 conception, that the sensory ring occurs at the anterior end of the somite, 

 places the somite limits as shown in the left half of Figure 4. 



2. Somite Reduction in NepJielis. 



An examination of the abbreviated somites at the ends of the body 

 should, as in the case of Glossiphonia, throw light on the question 

 whether somite limits and neuromere limits are identical. 



Giving attention first to the anterior end of the body, somite VIII., as 

 I place the somite limits (Fig. 4, right half), is the most anterior 

 unabbreviated somite in the body of Nephelis. Iu the case of somite 

 VII., ring 1 has disappeared. For, the nerve branch (x., somite VIII.) 

 which typically ramifies in ring 1 is wanting in somite VII. Moreover 

 the most anterior annulus of somite VII. is shown to be homologous with 

 annulus 2 of somite VI 1 1, by the fact that it contains the anterior nerve 



* No entire unabbreviated somite is shown in Figure 4, but the structure of a 

 typical somite may be learned from a study of the last six rings of the figure, 

 which represent unabbreviated the sensory ring and the two following rings of 

 somite VII., and the sensory and two preceding/ rings of somite VIII. 



