38 Arenicolidae 



ecaudate species all the segments are produced in this growing zone, 

 the activity of which becomes exhausted about the end of the post- 

 larval stage. In the caudate species, after the full number of 

 chaetiferous segments has been formed, the succeeding or tail-segments 

 are evidently produced at the anterior end of the tail ; for in this 

 region each segment is short from before backwards, while in the 

 middle and posterior regions of the tail the segments are longer, and, 

 in adult or late post-larval specimens, are subdivided into annuli. 

 In A. cristata the full number of tail-segments 38 to 40 is acquired 

 before the end of the post-larval stage. Examination of adult 

 specimens of A. marina and loveni, however, indicates that new 

 segments are continually being added to the tail at its anterior end, 

 and, as a consequence, the number of tail- segments may become 

 very large, e.g., in the tail of an example of A. loveni there were 

 175 septa, indicating as many segments. In A. marina there may 

 be 60 to 70 tail-segments, though usually there are fewer, owing 

 to losses posteriorly. 



In many of the tail-segments of all the caudate species of 

 Arenicola there is one annulus slightly larger and more deeply 

 pigmented than the rest, upon which distinctly larger epidermal 

 papillae are borne. Each of these larger annuli occupies a position 

 in the tail-segment corresponding roughly to that of the chaetiferous 

 annulus in the pre-caudal segments of the worm. 



Traversing the whole length of the mid-ventral line, in many 

 specimens of A. marina, pusilla, assimilis and loveni, there is a shallow 

 groove which marks the position of the ventral nerve-cord. A short 

 distance in front of the first chaetiferous annulus this groove unites 

 with the two metastomial grooves, which pass round the sides of 

 the peristomium (rnetastomium), in an antero-dorsal direction, to 

 the region of the prostomium (Fig. 53, p. 118). The metastomial 

 grooves indicate the course of the oesophageal connectives. In other 

 species the ventral and metastomial grooves are either faint or 

 absent. 



EXTERNAL APERTURES. The mouth, when the proboscis is with- 

 drawn, is a crescentic or semicircular transverse slit in the antero- 

 ventral region of the peristomium. It is overhung by a series of 

 papillae, which belong to the peristomium and form the upper lip, 

 dorsal and posterior to which the prostomium is situated. The anus 

 is posterior and terminal. 



The nuchal organ is a pro-curved, crescentic or U-shaped, ciliated 



