PHYSIOLOGICAL EFFECTS 131 



present was carefully checked and controlled. 

 Individual worms were isolated into separate 

 dishes containing various sorts of water and their 

 survival time was determined. Results from 

 such a test are graphically shown in Fig. 11. 



In this figure, the solid black columns give 

 the length of survival of marine flatworms isolated 

 into ordinary tap water. The total height at any 

 part of the figure shows the survival of similar 

 worms placed in the same amount of tap or pond 

 water in which other animals had lived, died and 

 disintegrated, or which had been otherwise con- 

 ditioned by association with living animals. 

 The stippled part of the diagram which lies 

 between the solid black and the open squares, 

 gives the survival of similar worms in very dilute 

 sea water which had the same amount of salt as 

 did the conditioned water. The height of the 

 figure at any point shows the percentage of worms 

 living at that time; the horizontal line gives 

 time in days except that the first 36 hours are 

 divided into 12-hour periods. 



The protective, conditioned water could be 

 made in various ways. One of the most effective 

 was to prepare a sort of worm soup from the ma- 

 rine flatworms themselves and to place this in a 

 collodion bag in distilled water until the salts 

 diffused out. Similar extracts of other marine 



