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 

 ao-ent 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."* 



'Ihis 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.f 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 ov.-n 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 iu most true 

 species of Scrtularia, Carapanularia, and Plumularia, and in aUied genera, the 

 tentacula or arms are destitute of cilia, and incapable of giving an impulsion to 

 the M>«<e»-."— Cyclopedia of Anat. and Physiology, Vol. i. p. 611. The observa- 

 tions of Mr Lister are equally decisive. Phil. Trans. 18-34, 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 diflfe- 

 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 diflTerence 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 supeiior order, it may also be disengag- 

 ed, although, if we may so speak, in a microscopic proportion, during the act of 



