440 F. W. GAMBLE AND J. H. ASHWORTH. 



botli A. marina and A. ecaudata (PI. 24, figs. 34, 35) ; (2) 

 the constant position of the otocyst ; (3) the vestigial seta ; 

 (4) the nerves given oil by the connectives ; and (5) the 

 presence of a segmental " giant-cell " in the cord just behind 

 the point of union of the commissures (see PI. 29, fig. 79). 



The debatable region in question extends from the hinder 

 border of the prostomium (as indicated by the nuchal groove) 

 to the annulus immediately in front of the first (adult) para- 

 podium. Laterally it is often marked by two sloping grooves, 

 the '' metastomial grooves" of Ehlers (1892), which indicate 

 the course of nerve connectives uniting the brain with the 

 ventral cord. It is crossed by a number of circular grooves, 

 which vary in number in different species. The skin of the 

 annulus between any two grooves is marked out into raised 

 areas like a mosaic. 



The position of the otocyst is indicated by an aperture 

 (A. marina) or by a slight depression (A. Grubii and A. 

 ecaudata), a little dorsal to the point of intersection of the 

 second of these grooves (counting from the prostomium) 

 with the metastomial groove. The otocyst is supplied by a 

 large nerve from the commissure, and a special development 

 of ganglionic cells occurs at the point of origin of the 

 otocyst nerve, extending slightly beyond and below this 

 point. The region between the prostomium and the second 

 groove is thus composed of two annuli bounding the mouth, 

 and containing the otocyst, otocyst nerve, and ganglion. This 

 region constitutes the peristomium. On this view the sugges- 

 tion made by Ehlers that the otocyst represents a pair of 

 modified peristomial cirri is extremely probable, but as the 

 early development of these organs has not been studied in 

 Arenicola, Aricia, Wartelia, or any other genus, the 

 suggestion can hardly be regarded as proven. 



The presence of a vestigial seta in post-larvse of A. 

 marina (Benham), A. ecaudata (Mesnil), which disappears 

 later, indicates the presence of a segment between the 

 peristomium and the first adult chaetigerous segment. We 

 have not been able to confirm the occurrence of this seta 



