200 LIGHT AND PROTOPLASM [Ch.VII 



currents in the water, but is a truly vital phenomenon. That 

 it is such is indicated also by the following curious behavior. 

 If swarm-spores which normally aggregate at 18° towards the 

 positive side of the drop, are suddenly brought to 18° from 30°, 

 they appear, for a moment, negative. Conversely, if swarm- 

 spores which normally aggregate at 30° towards the negative 

 side of the drop are suddenly brought from 8° to 30°, they ap- 

 pear, for a moment, positive. Thus, the immediately preceding 

 culture-temperature affects the sense of the response. 



The results obtained by Stbasburgbe have been in part 

 confirmed by other authors in other species. 



Groom and Loeb ('90, pp. 166, 172) state that in the case of the nauplii 

 of Balanus — " at a higher temperature, for instance 25° C, the phenomena 

 [of phototaxisj are run through more sharply and quickly than at a tempera- 

 ture of about 15°"; and again, "we often succeeded in suddenly changing 

 the sense of the heliotropism of the larva by a sudden change, of only a few 

 degrees, in the temperature of the water." This statement is unfortunately 

 so vague as to say little more than this, that temperature influences the 

 response. Massart ('91, p. 164) remarks, incidentally, that the flagellate 

 Chi-omulina is -|- phototactic at 20° C, but — phototactic at 5°C. Loeb 

 ('93, pp. 90, 96) obtained a result with Polygordius larvse and Copepoda 

 which seems, at first sight, the opposite of Strasburger's. Polygordius 

 larvse, negatively phototactic at 16°, were gradually cooled to 6°, at which 

 temperature they began to move rapidly towards the + side of the vessel. 

 As the temperature gradually rose they became — phototactic again. Indi- 

 viduals which were (abnormally) + phototactic at 17° to 24°, when raised 

 gradually to 29° became — phototactic. Sudden diminution of temperature 

 within the limits at which response occurs did not change the sense of their 

 response. Thus, negative individuals brought suddenly from 2-3° to 13° 

 remained negative. Exactly parallel results concerning the relation of 

 temperature and response were obtained by Loeb from Copepoda. 



All results may be harmonized in the expression : Diminu- 

 tion of temperature below the normal causes reversal of the 

 normal response ; elevation of the temperature to near the 

 maximum accelerates the normal response. The point of 

 light attunement varies with the temperature.* 



Not only light and heat, but also the concentration of the 

 medium affects light attunement. We are indebted to Loeb 



* It follows from these experiments that it is necessary in any phototac- 

 tic investigation to regard not only the intensity of the light, but also the 

 temperature. 



