386 MR. HANCOCK ON THE ANATOMY OF THE FRESHWATER 
little or on functional utility: consequently, these muscles are 
still present, though, as might be expected, not so fully developed 
as in those genera entirely deprived of this appendage. 
We have now gone through the whole of the muscular appa- 
ratus for retraction and propulsion, and to verify the use of the 
various sets of muscles, we must once more observe the animal 
while issuing from the cell. The first symptom indicative of the 
polype’s inclination to come forth, is the contraction of the parie~ 
tal muscles, causing the tunic in certain places to leave the walls 
of the cell, particularly towards the lower portion; on this the 
polype commences to move up the cell, and at the same instant 
the tube-retractors relaxing the inverted lips of the orifice, begin 
to be evolved, and as the contraction of the parietal muscles goes 
on, the polype advances upwards, and more and more of the tube 
is turned out, in the manner of the eversion of the horn of the 
common snail; at length the membranous cup makes its appear- 
ance, not doubled upon itself, but in an erect position — the 
margin first, just as the circle of sete is exserted in Bowerbankia. 
The cup at first is laterally compressed, having been packed 
longitudinally in the axis of the tube: the tips of the tentacles 
now emerge through the centre of this cup, and as they pass 
upwards, pressed together in a line side by side, its lateral folds 
give way, and by the time that the tentacular disc has reached 
the mouth of the cell, the cup is perfectly expanded. The 
muscles preventing the entire eversion of the tentacular sheath, 
may now be seen in action near the upper extremity of the tube, 
holding back the membranous sheath, and causing it to roll 
upon itself. The polype is now fairly above the mouth of the 
cell, and as the tentacles expand it has attained its greatest 
elevation; the cilia then commence to play, and all kinds of 
particles are hurried towards the mouth. 
The retraction of the polype is instantaneous, so rapid indeed, 
that it is quite impossible to follow with the eye, the actions of 
the muscles ;—such is the velocity with which this feat is per- 
formed, that from complete protrusion to invagination, nothing 
can be perceived but the settling of the polype upwards, after 
having apparently been dragged too far down the cell. It is not 
