I 



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 inversion 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 tor 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 inverted tube, yet I have never 

 seen the channel thus opened, although these muscles are never 

 otherwise than as represented in PI. 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 o, 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 



