137 
third, where there are either two rings (as in the 
Laminarian variety), or three (as in the littoral variety). 
The exact limits of the segments are not obvious, 
but from a consideration of the internal anatomy, it can 
be shown that the second groove behind each chetigerous 
annulus marks the posterior lmit of that segment. 
Internal septa, where present, are inserted at this level, 
not only in A. marina, but in all other species examined 
(see below).  Kach of the segments posterior to the third 
consists of the annulus which bears the parapodia, 
together with the three annuli in front of and the one 
behind it. The parapodia are therefore situated slightly 
behind the middle of the segment to which they belong. 
The third chetigerous segment consists, in the littoral 
variety, of four annuli, viz., the chetigerous one, two in 
front of it, and one behind it; while in the Laminarian 
variety this segment contains only three annuli, the 
middle one of which bears the sete. The extent of this 
third segment is easily determined, as it is delimited both 
anteriorly and posteriorly by internal septa. The second 
segment consists of three annuli, the middle one bearing 
the parapodia. ‘The first segment is composed of two 
rings, the anterior of which is chetigerous. This 
segment is bounded in front by the first diaphragm 
(fig. 2.) 
Between the first chetigerous annulus and_ the 
prostomium there is a region sub-divided into four rings, 
each of the first three of which may be again sub-divided 
into two; but usually there is little difficulty in recog- 
nising the four primary annuli (figs..5, 6). This 
region is composed of the peristomium and a_ body 
segment, the sete of which are minute, and disappear 
early. The evidence in support of this statement is 
derived from (1) an examination of post-larval stages, 
