Natural History of British Zoophytes. 24 1 
numerable cilia or miniature lamellae clothe the surfaces of their ten- 
tacula, and by their rapid vibrations drive a constant equable stream 
of water along one side, which returns along the other in an oppo- 
site direction ; and by this means the purposes of respiration are 
effected, and the nutrient fluid fitted for assimilation with the 
body. The cilia, to adopt the language of Professor Grant, " are 
disposed and moved in such a manner as that the streams which 
they produce in the surrounding water are driven along the one side 
of the tentaculum, from the mouth of the polypus, and on the other 
side of the tentaculum always towards the mouth of the polypus. 
And we never find that direction of their motion reversed, or that 
direction of the currents changed, by which, their respiration is ef- 
fected, and their food obtained. They are vibratile on the arms of most 
of the lower zoophytes, as sertulariae, pliiinulariae, serialairse, cellariae, 
flustrae, alcyonia, which keep their arms stiffly out in a regular cam- 
panulate form, while the currents flow to their mouth. When we 
watch the sides of the tentacula of these animals with attention, 
and by the aid of powerful glasses, we see the extreme rapidity of 
the movements, and the remarkable regularity of the form, disposi- 
tion, and motions, of those singular vibratile bodies. From the num- 
ber of them, exceeding sometimes 400,000,000 in a single animal, 
it is not probable that their extraordinary movements are the result 
anything analogous to cilia on the tentacula of the Hydra ; ( Org. Chem. p. 293,) 
and Dr Sharpey says, that in the form of polype " which exists in most true 
species of Sertularia, Campanularia, and Plumularia, and in allied genera, the 
tentacula or arms are destitute of cilia, and incapable of giving an impulsion to 
the water." Cyclopaedia of Anat. and Physiology, Vol. i. p. 611. 
Raspail maintains that there are really no cilia, but that the appearance 
of them is occasioned by currents of fluid aspired or drawn to and within the 
body, and expired or driven from it, and these currents are said to be produced 
by the difference of temperature between the fluid in the body and exterior to 
it. " A happy conjecture led me to consider these vibratory cilia as being mere- 
ly streams of a substance either inspired or expired, but at any rate of a diffe- 
rent density, and consequently of a different refractive power from the surround- 
ing medium." P. 293 " The cilia of a respiratory organ are, then, the effect 
of a difference of density between the water expired, and that in which the ani- 
mal swims. Now there is no difficulty in admitting that, since caloric is disen- 
gaged in the respiration of animals of a superior order, it may also be disengag- 
ed, although, if we may so speak, in a microscopic proportion, during the act of 
expiration in the Infusoria and the Molluscse. The difference, then, between the 
density of the water expired and that of the surrounding water, proceeds from 
a difference of temperature." P. 297. This explanation of Raspail's is complete- 
ly disproved by the observations of Professor Grant on the Beroe ; (Trans. 
Zool. Soc. i. p. 11.) and of Dr Sharpey on numerous animals Edin. New 
Phil. Journ. July 1835. 
