234 THE VITAL FUNCTIONS. 



each of the opposite currents occupying one^ 

 half of the circumference of the cyhndric cavity. 

 At the knots, or contracted parts of the tube, 

 slight eddies were noticed in the currents ; and 

 at each end of the tube the particles were seen 

 to turn round, and pass over to the other side. 

 In various species of Sertularice the stream 

 does not flow in the same constant direction ; 

 but, after a time, its velocity is retarded, and 

 it then either stops, or exhibits irregular eddies, 

 previous to its return in an opposite course ; 

 and so on alternately, like the ebb and flow of 

 the tide. If the currents be designedly ob- 

 structed in any part of the stem, those in the 

 branches go on without interruption, and inde- 

 pendently of the rest. The most remarkable 

 circumstance attending these streams of fluid 

 is that they appear to traverse the cavity of the 

 stomach itself; flowing from the axis of the 

 stem into that organ, and returning into the stem 

 without any visible cause determining these 

 movements. Similar phenomena were observed 

 by Mr. Lister in Campanularice and Plumularice. 

 In some of the minuter species of Crustacea, 

 the fluids have been seen, by the aid of the 

 microscope, moving within the cavities of the 

 body, as if by a spontaneous impulse, without 

 the aid of a propelling organ, and apparently 

 without being confined in membranous channels, 



