CIRCULATION IN POLYPI. 215 



and consequently capable of reflecting light, as 

 happens in many Polypi, we can have ocular evi- 

 dence, with the aid of the microscope, that a 

 general circular movement constantly takes place, 

 exactly analogous to the rotation of the fluid con- 

 tained in the cells and vessels of plants, and which 

 has been already noticed.* 



In the tubular stem of the Tuhularia incUvisa, 

 a current of particles is seen, strikingly resembling, 

 in the steadiness and continuity of its stream, the 

 rotatory movement of the sap in the cells of the 

 Chara. Its general course is parallel to the slightly 

 spiral lines of irregular spots on the surface of the 

 tube, ascending on the one side, and descending 

 on the other ; each of the opposite currents occu- 

 pying one-half of the circumference of the cylindric 

 cavity. At the knots, or contracted parts of the 

 tube, slight eddies may be noticed in the currents ; 

 and at each end of the tube the particles are 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 alter- 

 nately, 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 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 



* See page 45. 



