234 THE VITAL FUNCTIONS. 



scending on the other; each of the opposite 

 currents occupying one-half of the circumfer- 

 ence of the cylindric 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 SertularicB 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 obstructed in any part of the stem, 

 those in the branches go on without interruption, 

 and independently of the rest. The most re- 

 markable 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, 



