382 MR. HANCOCK ON THE ANATOMY OF THE FRESHWATER 
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 provided 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 
find them in action so long as the animal was retracted ; but we 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 invagi- 
nated state—unless we are to suppose that they are constantly 
employed in keeping open the channel, as suggested 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 Pl. V. 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 what- 
ever 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 con- 
siderable breadth ; the other, », is formed of only two or three 
fibres, which encircle the same tube at the insertion of the 
superior tube-retractors. The action of these sphincters cannot 
be mistaken : they effect the closing of the orifice on the retraction 
