218 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



(2) a comparison of this region in the adnlt 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 

 chsetigerous segment and the prostomiuni 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 achsetous, 

 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 

 chsetigerous segment. The anterior part of the region 

 under discussion is undoubtedly the peristomium, and 

 never bears setse. The otocysts, which in most other 

 Polychseta 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 is 

 making its appearance, the peristomium and the 

 achsetous body segment are each divided by a shallow 

 groove into two annuli, together therefore consisting of 

 four annuli. In two or three of these specimens the 

 metastomial grooves, which mark the track of the 

 oesophageal connectives, may be clearly seen uniting 

 ventrally near the middle of the third annulus (fig. 58). 

 The annulation of these post-larva3 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 prostomiuni and the first chsetigerous 

 annulus, the first two belong to the peristomium and the 

 third and fourth to a segment from which the setse have 

 disappeared in early life. Confirmatory evidence is 

 afforded by the giant nerve cells present in the nerve 

 cord. These cells occur singly or in couples near the 

 hinder limit of each segment, i.e., just behind the level 



