Freshwater Bryozoa, with descriptions of new Species. 187 



protrusion much in the same manner as in Plumatella 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 ; 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 

 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 oesophagus 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, PI. IV. figs. 1 r, r & 2 q, q, is the homologue of the 

 strong radiating muscles in Plumatella and Fredericella ; but the 

 fibres are much less numerous. Iu Paludicella they are only four 

 in number, and take their origin from the inner surface of the 



