No. I.] THE METAMERISM OF NEPHELIS. 35 



the first and second of their own segment, and the third of the pre- 

 ceding segment. The distribution is thus triannulate and dimeric." 



Passing to the head region, we find a number of interesting 

 modifications of the plan found in the body somites. Nerve 

 VI has three parts, "but they are no longer the precise equiva- 

 lents of 'anterior,' 'middle,' and 'posterior' nerves. What 

 before appeared as the dorsal branch of the posterior nerve 

 now appears as the middle nerve, supplying the same sense 

 organs as before and, in addition, the inner lateral sense organ 

 of segment V. The third nerve has no dorsal branch except 

 the short one to the outer lateral sense organ. It has two 

 main branches, however, one of which takes the place of the 

 ' middle ' nerve, the other that of the ' posterior ' nerve. The 

 first nerve alone remains the unchanged 'anterior' nerve. 

 The branch running to the inner lateral sense organ (2./.) of 

 segment V belongs, according to what we saw in typical seg- 

 ments, not to segment VI, but to segment V. 



" In segment V we find three nerves, but their composition and 

 distribution depart still further from the typical arrangement. 

 This nerve, as shown in PI. I [PI. IV here], gives off a number 

 of motor branches, and then passes to the outer lateral and 

 marginal sense organs and the labial organs of four rings (8-12). 

 It innervates then the first and second rings of its own segment, 

 and two rings (9-10) of segment IV. It corresponds then to 

 the ' middle ' nerve in the trunk region, but contains also fibers 

 belonging to three other nerves, namely, the ' posterior ' nerve 

 of the preceding segment, and the ' anterior ' and ' posterior ' 

 of its own segment. Just above and a little in advance of this 

 root appears another quite strong nerve, which rises and passes 

 forward over the lateral angle of the supra-oesophageal ganglia. 

 This nerve divides just in front of the head ganglia, sending 

 one branch to the inner lateral organ of segment IV, and the 

 other to the median organs of segments IV and V. This nerve 

 then corresponds to the dorsal sensory branch of a 'posterior' 

 nerve, and includes so far as it goes the fibers of two such 

 branches, for segments IV and V. 



" In segment IV we find only two nerves, one small motor, 

 corresponding to the 'anterior' nerve, and one large nerve 



