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- 
tides were seen to turn round, and pass over to the other side. In 
various species of Sertulariae, 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. 
| 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 
