310 ALFRED GIBBS BOURNE. 
entirely withdrawn). It is constantly changing its shape 
during life. It is hollow, the body-cavity being prolonged 
into it. Muscles attached to its inner wall and passing back- 
wards to the body-wall serve to withdraw it (see fig. 17), 
while if coelomic fluid is forced into it, it becomes much dis- 
tended. 
Setz.—The setz are very ordinary in shape (fig. 26), and 
bluntly pointed at either end. They present no remarkable 
features, nor do they differ, so far as I can see, to any extent 
from one another. They are as nearly as possible 0°5 mm. in 
length ; that is to say, they are very small as compared with 
the setze of most worms—as small as those of Microcheta. 
They are arranged in couples, the two setz in each couple being 
very near together. In a large-sized specimen which I measured, 
the circumference at about Segment xxv was 36°5 mm., 
the dorsal gap 18 mm., the lateral gaps 6°5 mm. each, and 
the ventral gap 5°55 mm. More anteriorly the dorsal gap 
increases very slightly at the expense of the lateral gaps.! 
Both seta couples are, as usual, absent from Segment 1; they 
are also absent in this worm from Segment 11, while in Seg- 
ment 111 the inner couples alone are present. I have never 
found any instance of any of the sete having accidentally fallen 
out or been lost ; and young sete (soies de remplacement), 
sO common in many worms, do not often occur; I think that 
they occur at certain particular seasons, when a new set of 
setee develop. There are no modified sete of any kind. The 
setee are so small that the position of neither the individual 
seta nor of the couple can be observed on an inspection of the 
inner surface of the body-wall. The position of the rows of 
seta couples is, however, clearly marked by the arrangement 
of the muscles (see below, Muscular system). 
1 The relative positions of the seta rows are of importance for systematic 
purposes in this group. My measurements are taken in the following 
manner: the worm is killed in-strong spirit, a ring of the body-wall, con- 
sisting of one or two somites, is cut off at about Segment xxv, cut through 
along one of the seta rows, and then flattened out, avoiding any stretching, 
and measured. 
