Freshwater Bryozoa, with descriptions of new Species. 185 
in around it the lips of the cell. The inferior set then taking up 
the work complete the mversion of the tube. Dr. Farre, how- 
ever, supposed that the opercular muscles were not merely for 
drawing the tube in after the retreating animal, but also for the 
purpose of closing the orifice. Professor Allman has pointed out 
the error of this opinion, and endeavoured to explain the closing 
of the orifice by the pressure of the fluid within the cell against 
the walls of the inverted tube. We shall directly see, however, 
that this theory is unnecessary, there being special muscles pro- 
vided for the purpose. Professor Allman is likewise disinclined 
to believe that the opercular muscles are really tube-retractors, as 
he supposes the muscles for drawing in the polype are sufficient 
for the purpose also of drawing in the tube. Were these latter 
muscles used to invaginate the tubular orifice of the cell, we 
should expect to find them in action so long as the animal was 
retracted ; but we have already seen, that when the polype is 
in this state, they are invariably relaxed and lie in a disordered 
undulating manner, perfectly at rest. The tube-retractors on 
the contrary are always tense and in vigorous action during the 
retracted state of the polype, evincing I think in a satisfactory 
manner that their function is to retract the tube and to maintain 
it in an invaginated state,—unless we are to suppose that they 
are constantly employed in keeping open the channel as sug- 
gested by Professor Allman. They will certainly have a tendency 
to pull asunder the walls of the mverted tube, yet I have never 
seen the channel thus opened, although these muscles are never 
otherwise than as represented in Pl. IV. fig. 2, when the polype 
is retracted. And moreover the tips of the tentacles, as exhibited 
in this figure, are frequently doubled down, showing that the 
tentacular sheath must be to some extent relaxed, and that 
there is no stress whatever on it, as there would be were the 
polype-retractors used to draw in the orifice. 
The fourth set of muscles to be described is for closing the 
orifice. This set is composed of two sphincters: one, fig. 2 0, of 
these is made up of several fibres passing round the tube at the 
place of insertion of the inferior tube-retractors, and is of consi- 
derable breadth ; the other, p, is formed of only two or three 
fibres, which encircle the same tube at the insertion of the supe- 
rior tube-retractors. The action of these sphincters cannot be 
mistaken ; they effect the closing of the orifice on the retraction 
of the animal; being at the same time antagonistic to the open- 
ing tendency of the tube-retractors, which, diverging from their 
insertions, must in some measure bring the walls of the inverted 
tube asunder. These sphincters are not readily distinguished, but 
having seen them in several individuals, I have not the slightest 
doubt of their existence. Deeming however that it would be 
