OF POLYPES. 
43 
As to the purpose of the circulation in the animal’s economy, it 
appears, from the experiments of Mr Lister, “ to be the great 
agent in absorption , and to perform a prominent part in the ob- 
scure processes of growth ; and its flow into the stomach of the 
polypi seems to indicate that in the very simple structure of this 
family it acts also as a solvent of the food. — The particles car- 
ried by it,” continues Mr Lister, “ present an analogy to those 
of the blood in the higher animals on one side, and of the sap 
of vegetables on the other. Some of them appear to be deriv- 
ed from the digested food, and others from the melting down 
of parts absorbed ; but it would be highly interesting to ascer- 
tain distinctly how they are produced, and what is the office they 
perform, as well as the true character of their remarkable acti- 
vity and seemingly spontaneous motions ; for the hypothesis of 
their individual vitality is too startling to be adopted without good 
evidence.”* 
This sort of circulation is not to be confounded with those 
aqueous currents which flow over the surfaces of the external 
organs of the ascidian polypes. - }* It has been already stated 
* Phil. Trans. 1834, p. 377. 
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 
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. The observa- 
tions of Mr Lister are equally decisive. Phil. Trans. 1834, p. 377. 
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 
