139 
of the parapodia (see below). Just behind the meeting 
pot of the csophageal 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 sete have disappeared. The 
first chetigerous 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 shght 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 off 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 papille are distinctly 
larger than on the other annul. Each of these larger 
annuli occupies a position in the segment roughly corres- 
ponding to that of the chietigerous annulus in the pre- 
caudal segments of the worm. ‘These larger annuli and 
their papille are best seen near the middle of the tail, 
behind that point the distinguishing characters above 
