230 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



nephrostonie and binds the latter to the body wall (figs. 

 23, 24 and 25). 



Each bundle of notopodial setse 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 setee 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 entirely withdrawn into the setal sac (fig. 24).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 coelomic 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 

 oesophageal 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 p. 19). 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. Each diaphragm is covered 

 on both its faces by an endothelium composed of flattened 

 cells between which is a thin layer of connective tissue 



