310 
Sfiulies OH P((Jicalus 
collar-membraue in which they are inserted is partly torn away and 
lies external to the inverted vesica. 
Following upon a moderate flexing of the dilator, whereby it forms 
roughly a right angle with the basal plate (Text-fig. 5; PI. Ill, fig. 2) 
a third movement takes place. This movement concerns the expulsion 
of the vesica, and it is due to the flow of coelomic fluid out of the body 
cavity into the vesica, all of the retractor muscles being the while relaxed. 
The expulsion of the coelomic fluid which renders the sack turgid, is due 
to powerful contractions of the trunk muscles whereby the fluid is 
pressed out of the body into the vesica. It is possible that there are 
fine muscles present which serve to constrict the channel into the sack 
Fig. 5. Pediculus Ituma/ius q. Posterior part of the abdomen, showing the first stage 
in the protrusion of the eopulatory apparatus, when the male attacks the female. 
Only the dilator protrudes and beneath it is seen the collar-like membrane which 
forms a sheath for the transit of the vesica, etc. Drawing from preserved specimen. 
thus maintaining it erect, but if they exist they still remain to be dis¬ 
covered. It seems likely that there are no such muscles, and that the 
sack remains inflated once it has been filled with fluid, the turgidity 
being maintained throughout by the moderately sustained contraction 
of the muscles which reduce the space for the fluid in the body. When 
the vesica is fully extruded, the point of the dilator is fully depressed by 
the flexor muscles already referred to. 
In the retracted apparatus (PI. Ill, fig. 1), the point of the penis is 
directed outward toward the cleft in the dilator {DU. cl.), the result 
being that when the vesica begins to issue (PI. Ill, fig. 2, Pen.) the point 
of the penis promptly emerges from the cleft, its point being surrounded 
