J. A. KITCHING 



needed, in material favourable for this purpose. It is possible that there may be con- 

 siderable specific variations in the part played by this form of activity, and in any 

 case the observation needs repeating. Peristalsis of the tentacles would account for 

 the fact that a feeding suctorian becomes nearly spherical when full, as though 

 pumped up. It would no doubt also help in the passage of lumps of material. On the 

 other hand the expansion of the body surface, if accompanied by a resistance to 

 inward collapse, would very conveniently provide a suction. It might take place 

 in the way suggested by Mitchison (1952) for the surface of dividing sea-urchin 



eggs. 



The contractile vacuole plays an important though limited part in the process of 

 feeding (Pestel, 1931 ; Kitching, 1951 and 19526; Rudzinska and Chambers, 1951 ; 

 Hull, 1953). It operates at a much faster rate during feeding, and so serves to remove 

 some of the water brought in as part of the substance of the prey. During a meal the 

 suctorian swells much less than the prey shrinks, and the difference is accounted for 

 by the extra activity of the contractile vacuole, (Kitching, 19526; Hull, 1953). If the 

 body volume were constant, and the surface of the body were rigid, extra activity of 

 the contractile vacuole would presumably produce a suction which might account 

 for the process of feeding. However, the body does in fact get bigger, and this cannot 

 be explained by vacuolar activity. 



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Collin, B. (1912). Etude monographique sur les Acinetiens. II. Morphologie, 



Physiologie, Systematique. Arch. £ool. exp. gen. 51, 1-457. 

 Dragesco, J. and Guilcher, Y. (1950). Sur la structure et le fonctionnement des 



tentacles d'Acinetiens. Microscopie 2, 17-24. 

 Hartog, M. (1901). Notes on Suctoria. Arch. Protistenk. 1, 372-374. 

 Hull, R. W. (1953). Observations on Suctoria: contractile vacuole rate changes 



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