SENSORY EPITHELIUM OF MICROSTOMA CAUDATUM. 



53 



are: (a) Radical changes of temperature; (b) rapid accumulation 

 of bacteria. That the first is not the greater factor is shown by 

 the fact that in one aquarium, in which for some reason bacteria 

 did not accumulate, specimens remained from December 8 until 

 February 3 and still gave reactions which showed that they 

 had not lost their physiological tone. Text-figure io-a indicates 

 the path of such a specimen when placed in fresh pond water 

 containing near its center a drop of eosin .05 per cent, salt 

 solution. It would be w r ell to compare this figure with text- 

 figure io-& which indicates the path of a specimen, with reference 

 to a drop of .05 per cent, salt solution, which had been kept in an 

 aquarium only five days, but upon which vessel a thin glea of 

 bacteria had collected. As can be seen the latter specimen was 

 indifferent to the salt solution. 



This observation led us to test the reactions of two lots of 

 specimens taken from fresh aquaria: 



The first lot we placed in a watch glass with some of the brown 

 glea of an old aquarium for two hours. These specimens showed 

 plainly that they had lost their physiological tone. Text-figure 

 n-a indicates the path of one of these specimens when placed 

 in .05 per cent, salt solution containing a drop of fresh water in 



FIG. 10. Diagrams indicating the paths of specimens when placed in fr.'sh wate 

 and encountering a drop of .05 per cent, salt solution. The dotted line indicates 

 the contour of the drop of .05 per cent, salt solution surrounded by fresh water. 



the center. It can be seen that the specimen passed from one 



medium into the other six times and did not react to them at all. 



The second lot we placed in a watch glass and then put this 



