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(2) a comparison of this region in the adult and in post- 
larval stages, and (3) a consideration of the position of 
certain giant nerve cells present in the adult. 
In all post-larval stages the region between the first 
chetigerous segment and the prostomium is clearly 
divided into two parts by a groove (figs. 57, 58). The 
posterior of these is a body segment, which in all the 
seventeen post-larval stages I have examined is achetous, 
but in which a small seta was observed by Benham. This 
seta, though it disappears early, indicates that there is a 
distinct trunk segment in front of the first adult 
chetigerous segment. The anterior part of the region 
under discussion is undoubtedly the peristomium, and 
never bears sete. The otocysts, which in most other 
Polycheta in which they are present are peristomial, may 
be seen near its anterior margin (figs. 56, 57). 
In post-larval stages in which the annulation 1s 
making its appearance, the peristomium and_ the 
achetous body segment are each divided by a shallow 
groove into two annul, together therefore consisting of 
four annul. In two or three of these specimens the 
metastomial grooves, which mark the track of the 
cesophageal connectives, may be clearly seen uniting 
ventrally near the middle of the third annulus (fig. 58). 
The annulation of these post-larvee corresponds, there- 
fore, to that of the adult (cf. figs. 6, 58). It is evident 
from the above facts that of the four annuli in the adult 
between the prostomium and the first chetigerous 
annulus, the first two belong to the peristomium and the 
third and fourth to a segment from which the sete have 
disappeared in early hfe. Confirmatory evidence is 
afforded by the giant nerve cells present im the nerve 
cord. These cells occur singly or in couples near the 
hinder limit of each segment, 7.e., just behind the level 
