Freshwater Bryozoa, with descriptions of new Species. 187 
protrusion much in the same manner as in Plumatelia and Fre- 
dericella. In these however there is some little difficulty, the 
cells being continuous ; but in Paludicella, in which they are all 
separated, this act can be clearly understood. I have certainly 
observed in Plumatella and Fredericella the appearance of divi- 
sions here and there, forming as it were the cells into groups or 
systems, but nothing to warrant the belief that each cell is iso- 
lated. 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 inconvenient 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 the 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, Fredericella, and Alcyonella it 
is always straight ; m 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 maintaims the protruded animal in its 
position, it is more than probable that the same power would be 
sufficient to perform the act of protrusion. From the movements 
of the alimentary canal it is pretty evident that it has the power 
of straightening itself: but when quite straight only a portion 
of the tentacles would be protruded beyond the cell; and here 
they would remain, for it is very clear that whether straight or 
bent, the alimentary canal will displace the same quantity of fluid, 
and that there would be no increase of pressure to force the ani- 
mal upwards. It is at the moment when the alimentary canal is 
being straightened that the parietal muscles come into play, and 
compel the animal to rise above the cell: these acts are perfectly 
simultaneous. The protrusion therefore of the polype with a 
bent cesophagus and intestine is effected in the same manner as 
that in which these organs are straight ; only that in the former 
it is accompanied with the straightening of the alimentary canal. 
The sixth and last set of muscles to be described is for the 
purpose of preventing the entire eversion of the tentacular sheath, 
This set, Pl. IV. figs. lr, r & 2 g, q, 1s the homologue of the 
strong radiating muscles in Plumatella and Fredericella ; but the 
fibres are much less numerous. In Paludicella they are only four 
m number, and take their origin from the inner surface of the 
