I, 



O. I.] THE METAMERISM OF NEPHELIS. 49 



body metamere. Proceeding forwards, the 8th annulus has an 

 intermuscular nerve ring, innervated as has just been described 

 from the succeeding metamere, VI. 



The inner, or median, branch of nerve V corresponds to the 

 anterior lateral nerve of a body somite and innervates a few- 

 ventral sensillae on the 7th annulus, the outer lateral sensillae 

 of annulus 6, the ventral portion of the intermuscular nerve 

 ring in annulus 5, and thence passing forwards innervates the 

 labial sense organs on the ventral margin of the oral sucker. 

 The outer branch, corresponding to the posterior lateral nerve, 

 rises sharply to the dorsal side (PI. VII, Fig. 16), innervates the 

 sensillae in the 7th and 6th annuli, and sends a branch to the 

 intermuscular nerve ring in the 5th annulus. The 6th, 7th, 

 and 8th annuli, then, form metamere V, and again we find the 

 dimeric and triannulate distribution found in Clepsine and in 

 the normal body metamere of Nephelis. From this metamere 

 forward the distribution is simpler but readily referable to the 

 body metamere. Annulus 5 is, as has been described, very 

 narrow and situated in the plane of flexion, yet it represents 

 metamere IV, for it contains an intermuscular nerve ring 

 innervated from the succeeding somite, and nerve IV gives off 

 a dorsal branch which, quickly dividing, innervates dorsal sen- 

 sillae and the third pair of eyes in this annulus, while the ven- 

 tral branch goes forward to innervate some of the lateral labial 

 sense organs. The persistence of this annulus in the plane of 

 flexion is a striking instance of the stability of the'ist annulus 

 of the metamere. Reduced, by its position, to the narrowest 

 annulus in the animal, so narrow that the eye belonging to 

 it has been forced partly outside of it into the broad 4th 

 annulus, it retains not only the characteristic features of the 

 1st annulus, but also the intermuscular nerve ring belonging 

 to the posterior annulus of the normal body metamere. 

 Annulus 4 is broad and bears two rows of large sensillae 

 on its dorsal surface. It represents metamere III. Nerve III 

 divides as it enters the annulus, sending off a dorsal branch, 

 which soon divides, one branch going to the sensillae of the 

 annulus, the other innervating the second pair of eyes. The 

 ventral branch goes to labial organs on the dorso-lateral margin. 



