240 Natural History of British Zoophytes. 



sels — in any zoophyte, but in many of the hydroida it has been 

 discovered that there is a continuous and uniform current of a fluid, 

 containing granular bodies which have themselves a rotatory motion, 

 within the tubular portions of the horny polypidom. Cavolini first 

 detected this sort of circulation, which is very similar to what has 

 been observed in the Chara and other plants, in the Sertularia ; and 

 recently Mr Lister has confirmed this discovery, and ascertained 

 the existence of the same phenomenon in almost all the genera of 

 the order. The 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 with- 

 in 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 irregular spots on the surface of the tube, ascending on the 

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

 rents occupying one-half of the circumference of the cylindric ca- 

 vity. At the knots, or contracted parts of the tube, slight eddies 

 were noticed in the currents ; and at each end of the tube the par- 

 ticles 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 irregular eddies, previous to its re- 

 turn 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 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." * 



This sort of circulation is not to be confounded with those 

 aqueous currents which flow over the surfaces of the external or- 

 gans of the ascidian polypes, t It has been already stated that in- 



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

 p. 150. and Ent. Mag. Vol. iii. p. 174 — Dr Sharpey appears to have discovered 

 that the currents are produced by vibratile cilia.— Edin. New. Phil. Journ. for 

 July 1835. 



f Dr Grant repeatedly asserts that the tentacula of the hydraform polypes are 

 also ciliated, and I would not have dared to controvert this statement, although 

 my own observations had long ago satisfied me of its incorrectness, had it not 

 been at variance with the observations of others who have especially directed 

 their attention to the subject. Raspail states that he was not able to discover 



