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tions :—(1) an anterior bearing parapodia, but no gills, 
2) a middle region bearing both parapodia and gills, and 
(3) a posterior portion or tail, which has neither sete nor 
gills. Wach of these parts is composed of a number of 
segments, but the segmentation is somewhat obscured by 
the sub-division of each segment into a number (generally 
five) of rings or annuli, 
One of the annuli of each segment is larger than the 
rest and bears the parapodia. ‘The latter may be best 
seen in the middle branchiferous segments. ‘They are 
obviously much reduced compared with those of eres, 
and their two rami are different in shape and more dis- 
sociated than they are in .Vere’s. The notopodium is a 
small conical elevation situated dorso-laterally, on the 
rounded apex of which is the mouth of the setal sac from 
which the tips of a small pencil of hair-like sete project 
toa greater or lesser extent (fig. 19). The neuropodium 
is a muscular ridge traversed dorso-ventrally by a narrow 
slit, the mouth of the setal sac, in which are situated in 
a linear series the numerous chet or crotchets. The 
first few neuropodia are small, but in the posterior part of 
the branchial region they are well developed and extend 
ventrally until those of the right and left sides almost 
meet in the mid-yentral line. Each has the appearance 
of a pair of closely applied tumid lips between which only 
the tips of the curved cheetee project. There are no cirri 
or acicula in the parapodia of Arenicolu. Fora detailed 
description of the sete, see below. 
There ave thirteen branchiferous segments, and in 
the region in front of the first gill six chetigerous 
segments may be recognised. The interval between two 
cheetigerous annuli is sub-divided into four rings, except 
between the first and second chetigerous annuli, where 
there are only two rings, and between the second and 
