External Characters of Arenicola 37 



out chaetae in the adult. In post-larval stages of A. marina (PL X, 

 Fig. 27) and ecaudata (PL XI, Fig. 34) the region between the 

 prostomium and the first ordinary chaetiferous segment is sub- 

 divided by a groove into two parts. The anterior and usually 

 rather smaller portion is undoubtedly the peristomium ; it never 

 bears chaetae, but the paired statocysts may be seen near its anterior 

 margin. The posterior of the two parts is, in the post-larval stages 

 which the writer has examined, achaetous, but a chaeta has been 

 observed in this segment, in either A. marina or A. ecaudata, by 

 Professors Ehlers, Benham, Mesnil and Fauvel, a fact which demon- 

 strates that this is a true segment. 1 In later post-larval stages, in 

 which the annulation is making its appearance, the peristomium and 

 the segment in question become subdivided into secondary rings, so 

 that, henceforward, the segmentation of this region, like that of the 

 rest of the body, is obscured; but it frequently happens that the 

 original groove between the peristomium and the segment under 

 consideration remains more obvious than any of those subsequently 

 formed, and is recognisable even in the adult (Fig. 54, p. 118). 

 Dr. E. S. Lillie 2 does not agree with the interpretation given 

 above, because he could not find, in young A. cristata, a septum 

 corresponding to the achaetous segment. But the considerable 

 length and subdivision of, and the presence of two apparently 

 segmental pigment-bands in, the region in question (PI. X, Fig. 30) 

 suggest that it includes the peristomium and another segment. 

 The composition of this region is probably constant throughout 

 the family. 



In A. ecaudata and branchialis the parapodia are continued to 

 the posterior end of the animal ; but in A. marina, pusilla, assimilis, 

 loveni, cristata and glacialis there is a " tail " in which parapodia and 

 gills are not present. The tail is marked with a number of slight 

 constrictions, which indicate the boundaries of the somites and 

 correspond to internal septa. The number of tail-segments varies 

 considerably in different individuals of the same species, because 

 several segments, or even the major portion of the tail, may be thrown 

 off on irritation. 



During development, new chaetiferous segments are formed 

 immediately in front of the terminal segment or pygidium. In the 



1 Evidence confirmatory of this interpretation is afforded by the arrangement 

 of the giant nerve-cells (Ashworth, Liverpool Mar. Biol. Comm. Mem. xi 

 (1904), p. 11). 



2 Mitt. Zool. Stat. Neapel, xvii (1905), p. 358. 



