150 
nephrostome and binds the latter to the body wall (figs. 
23, 24 and 20). 
Kach bundle of notopodial sete is enclosed in a sac 
to the inner end of which are attached (1) a single re- 
tractor muscle strand, which is inserted into the body wall 
at the side of the nerve cord, and (2) six to ten protractor 
muscles, which are inserted into the body wall at the level 
of the setal sac. Contraction of the latter causes pro- 
trusion of the setze beyond the lips of the setal sac, while 
by shortening of the retractor muscle they may be almost 
entirely withdrawn into the setal sac (fig. 24). 
There is a strong sheath of muscle fibres attached to 
the pharynx and to the neighbouring body wall (fig. 23). 
Contraction of these muscles produces withdrawal of the 
pharynx. : It is probably by the help of the ceelomic fluid, 
which can be collected here and subjected to pressure, that 
the pharynx is protruded. | 
The prostomium is provided with a small sheet of 
retractor muscle arising from the musculature around the 
cesophageal connectives and inserted into the ventral sur- 
face of the brain and hinder edge of the nuchal organ 
(fig. 34). 
The position of the three anterior diaphragms and the 
dorsal and ventral mesenteries in the second segment has 
already been noticed (see above). Each of these dia- 
phragms is perforated by numerous rounded, usually oval, 
apertures which are best seen in the third diaphragm 
where they are moderately close together and about ‘02 
to ‘(03 mm. in diameter (fig. 45). These openings permit 
the passage of the coelomic fluid and its cells but prevent 
all but the smallest ova and spermatogonia from passing 
into the anterior segments. Hach diaphragm is covered 
on both its faces by an endothelium composed of flattened 
cells between which is a thin layer of connective tissue 
