MAEINE BIOLOGICAL STATION AT PORT ERIN. 115 



instance with sea-water, and fresh-water was then added 

 drop by drop till the specific gravity, as tested by the 

 behaviour of small vortex rings, was identical with that 

 of the sea-water in which the Polycarpse were living. 



" Results : — (1) At the moment of contraction of the 

 body water was driven violently out by the branchial and 

 less violently out by the atrial aperture. 



" (2) When undisturbed a slow steady current enters by 

 the branchial aperture. 



" (3) When undisturbed the current at the atrial 

 aperture is slow, and is sometimes inwards and sometimes 

 outwards. It was observed to flow thus in opposite 

 directions, while the inward current at the branchial 

 aperture remained constant and steady. 



" (4) When a moderately strong mixture of the paint 

 was used, the entrance of it by either aperture was 

 followed (sometimes immediately, but at other times only 

 after a considerable quantity had been inhaled) by sudden 

 contraction of the body and expulsion of water by both 

 apertures, but chiefly by the branchial. In some indi- 

 viduals, at least, this contraction followed more rapidly 

 upon entrance of the paint by the atrial aperture than by 

 the branchial. 



" These results would seem to verify the suggestion of 

 Prof. Herdruan* that the occurrence of tentacles about 

 the atrial aperture is connected with an occasional inhal- 

 ation by that aperture, the tentacles serving to detect 

 impurities in the water so inhaled, the stimulation being 

 followed by contraction of the body and expulsion of the 

 impure water." 



* British Association Report, Edinburgh, 1892, p. 788 ; and Bulletin 

 Scientifique de la France et de la Belgique, t. xxv., p. 56, 1893. 



