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by folds of the issuing vesica. The distortion in the form of the dilator 
due to pressure from the vesica has already been noted, that this pressure 
is very considerable is clear; we have one specimen in which a fracture 
of one of the rods of the dilator distally appears attributable to this 
cause. 
Finally, the whole vesica disappears from the dorsal surface of the 
basal plate (Text-fig. 2); the neck of the sack is the last part to emerge 
(when inverted, it lies adjacent to the cleft in the dilator upon the ventral 
surface of the basal plate); the non-dentate or rugose portion (Text-figs. 
2, 3, r. p. s.) is clearly seen in longitudinal sections. 
The mechanism whereby retraction is effected. The vesical musculature 
is shown in Text-figs. 2, 3 and 4, and keeping in mind the description 
given of the process of expulsion of the vesica we obtain a fairly clear 
conception of how retraction is effected. To begin with, any pressure 
which may have been maintained upon the coelomic fluid in the vesica 
woidd have to be suspended, the flexor muscles of the dilator and the 
protractors of the basal plate would have to be successively relaxed. 
As is well shown in Text-flg. 2, the vesica is supplied with numerous 
very fine and long muscle fibres which arise around the anterior rounded 
margin of the basal plate upon its dorsal surface. These muscles run 
parallel to each other dorsally to the basal plate, and some of them, on 
entering the vesica, diverge in smaller bundles running respectively to 
the walls of the vesica, the statumen penis and penis base. The number 
of fibres is actually larger than figured, some fifty-six fibres having been 
counted in a cross section of the vesica. It is clear that when these 
muscles contract they will retract all the parts in which they are inserted, 
thereby causing the vesica to empty back its coelomic fluid into the 
body cavity while the vesica gradually becomes invaginated. It is 
possible that some fibres may give a certain independence of movement 
to the penis. 
In dissections of the retracted copulatory apparatus, the retractor 
muscles of the vesica form a roimded mass, recalhng a heap of string, 
lying dorsaUy upon the retracted vesica above the anterior end of the 
basal plate. In sections, the fibres run in different directions (Text- 
fig. 3, M. retr. V. p.), and from sections alone it would be impossible to 
explain their mode of action; these muscles constitute the so-called 
“Bulbus Muskulatur des Penis” of Miiller (1915). The best picture of 
their arrangement is conveyed by Text-fig. 4 which represents a drawing 
from a dissection of the retracted copulatory apparatus. In this case 
the basal plate {B. PI.) was cut across and the muscles running from its 
