SEA-FISHERIES LABORATORY. 217 



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 chsetigerous 

 annulus marks the posterior limit of that segment. 

 Internal septa, where present, are inserted at this level, 

 not only in A. marina, but in all other species examined 

 (see p. 228). Each 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 chsetigerous segment consists, in the littoral 

 variety, of four annuli, viz., the chaetigeroiis 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 seta?. 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 ehsetigerous. This 

 segment is bounded in front by the first diaphragm 

 (fig. 2.) 



Between the first chaetigerous 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 setae of which are minute, and disappear 

 early. The evidence in support of this statement is 

 derived from (1) an examinaticn of post-larval stages, 



