384 MR. HANCOCK ON THE ANATOMY OF THE FRESHWATER 
their arrangement, neither could I determine their exact relation- 
ship to the tunic—whether they were attached to it by their 
extremities only, as supposed by Dr. Farre, or connected with it 
throughout their entire length. Professor Allman appears to be 
of the latter opinion, and certainly I saw nothing in confirmation 
of that expressed by Dr. Farre ; though I am not sure that the 
extremities are not attached to the cell-walls, thus giving to these 
muscular belts fixed points of action. Howsoever this may be, 
these parietal muscles undoubtedly have the power of contracting 
the tunic, and so lessening the space within which the polype is 
confined, the contained fluid is made to press on the surface of 
the polype, constraining it to pass upwards, and thus to effect its 
protrusion much in the same manner as in Plumatella and 
Fredericella. In these, however, there is some little difficulty, 
the cells being continuous ; but in Palwdicella, in which they are 
all separated, this act can be clearly understood. I have certainly 
observed in Plumatella and Fredericella the appearance of divisons 
here and there, forming, as it were, the cells into groups or systems, 
but nothing to warrant the belief that each cell is isolated. It 
might, therefore, be thought that protrusion of a few of the polypes 
would necessitate that of the others, or at least would cause an 
imconvenient pressure on the other members belonging to the 
same group. 
It is difficult to arrive at a full explanation of the propulsion 
of the polype in these cases ; but there can be no doubt that in 
them, as in other Bryozoa, the contraction of the tunic is the 
sole agent. Dr. Farre believed that the act of protrusion did not 
so much depend on the contraction of the tunic, as on the 
straightening of the alimentary canal, which, in the marine spe- 
cies and in Paludicella is doubled upon itself when the polype 
is retracted. But in Plumatella, Predericella, and Alcyonella, it 
is always straight ; in these genera, therefore, protrusion cannot 
in the least be assisted by the alimentary canal. Professor All- 
man has referred to this fact to prove the error of Dr. Farre’s 
opinion ; and, indeed, if it be allowed, and I suppose it must, that 
the pressure of the fluid maintains the protruded animal in its 
position, it is more than probable that the same power would be 
