SEA-FISHERIES LABORATORY. 219 



of the parapodia (see p. 257). Just behind the meeting 

 point of the oesophageal connectives there is a single 

 giant cell, which is therefore situated either in the 

 posterior part of the third or, more usually, in the fourth 

 annulus (fig. 52). This cell, whose position near the 

 hinder border of the somite corresponds with that of the 

 giant cells of other somites, must therefore belong to the 

 segment from which the seta? have disappeared. The 

 first chsetigerous segment of an adult Arenicola is, there- 

 fore, really the third segment, as it is preceded by a 

 segment bearing a giant cell and vestigial seta, and this 

 again by the peristomium. 



The tail, which is without parapodia and gills, varies 

 considerably in different specimens. It is usually marked 

 by a number of slight constrictions, best seen when the 

 tail is stretched, which indicate the boundaries of the 

 somites and correspond in position to the internal septa 

 (fig. 1). There may be as many as seventy segments, but 

 in most specimens there are fewer as the worm has a ten- 

 dency to throw oft the last few segments when irritated. 

 New segments are apparently formed at the anterior end 

 of the tail region. Here the segments are short from 

 before backwards, but further back they are longer and 

 sub-divided into a number of annuli, as many as ten in 

 some of the last segments. Near the posterior limit of 

 each segment in the mid-tail region there is an annulus 

 slightly larger and often more deeply pigmented than the 

 rest, and upon which the epidermal papilla? are distinctly 

 larger than on the other annuli. Each of these larger 

 annuli occupies a position in the segment roughly corres- 

 ponding to that of the chaetigerous annulus in the pre- 

 caudal segments of the worm. These larger annuli and 

 their papilla? are best seen near the middle of the tail, 

 behind that point the distinguishing characters above 



