42 STRUCTURE AND PHYSIOLOGY 



is very similar to what has been observed in the Chara and other 

 plants, in the Sertularia; and recently Mr Lister has confirm- 

 ed this discovery, and ascertained the existence of the same 

 phenomenon in almost all the genera of the order. 'J he result 

 of his curious observations is thus summed up by Dr Roget. 

 " In a specimen of the Tubularia indivisa, when magnified one 

 hundred times, a current of particles was seen within the tubular 

 stem of the polype, strikingly resembling, in the steadiness and 

 continuity of its stream, the vegetable circulation in the Chara. 

 Its general course was parallel to the slightly spiral lines of ir- 

 regular spots on the surface of the tube, ascending on the one 

 side, and descending on the other ; each of the opposite cur- 

 rents occupjing one-half of the circumference 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 Sertularise, 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 irregu- 

 lar eddies, previous to its return in an opposite course ; and so 

 on alternately, like the ebb and flow of the tide. If the cur- 

 rents 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 stem, without any visible cause de- 

 termining these movements." * 



The power which sets in motion and maintains this current 

 is yet undiscovered. Professor Grant asserts that it depends 

 on the action of minute vibratile cilia, — " the common agents 

 of all analogous movements in the lowest tribe of animals, ""f* — 

 but no direct observation has confirmed this explanation, which, 

 it will be observed, is founded on analogy only, and it has this 

 in opposition — that the non-existence of cilia in the external or- 

 gans of the zoophytes in question has been distinctly proved. 



* Bridgew. Treat. Vol. ii. p. 233. See also Tiedemann's Comp. Pliysiol. 

 p. 150, Ent. Mag. Vol. iii. p. 174: and Grant's Outlines of Comp. Anat. p. 

 429-30. 



t Oiitliiu's ()(' Coinj). Aunt. p. 430. 



