FRANDSEN. — REACTIONS OP LIMAX MAXIMOS. 207 



will have a correspondingly lower optimum. May it not be that every 

 organism will respond positively to a certain range of light intensities and 

 negatively to another range of intensities which is greater ? The nature 

 of the phototaxis may sometimes be gradually changed by organisms 

 becoming acclimated to new conditions. Verworn ('89, pp. 47-49) found 

 that a culture of the diatom Navicula brevis, which ordinarily is negatively 

 phototactic to very weak light, became positively phototactic when reared 

 for several weeks near a window. Groom und Loeb ('90, pp. 166-167) 

 found that young Nauplius larvae of Balanus which were at first positively 

 phototactic to daylight became negatively phototactic later in the day, 

 probably as the result of the accumulated effects of this exposure. 



The character of the light responses, as was the case with geotaxis, 

 depends also to a certain extent on other external conditions, such as 

 those of temperature, the states of density and pressure, and various 

 chemical influences. Polygordius larvae, when gradually cooled from 

 16.5° C. to 6° C, were found by Loeb ('93, pp. 90-96), to change from 

 a negative to a positive phototaxis. Like results were obtained by him 

 from Copepoda. When the temperature was raised from 6° C. to 16° C, 

 the animals again became negative. Increasing the density of sea-water 

 by the addition of sodium chloride produced a change from a negative to 

 a positive response, thus acting like diminished temperature. Engelmann 

 ('82, pp. 391-392) showed the apparent phototactic response of chlor- 

 ophyllaceous ciliates to be really a chemotactic attraction for oxygen, 

 which chlorophyll can produce only in the light. These facts make it 

 important in any study of light response to consider other possible influ- 

 ences, and above all to take account of the strength of the stimuli used. 



Davenport and Perkins ('97) found that the slug (Limax maximus) 

 responded with marked precision to the varying stimuli of gravity at 

 different angles of inclination of the glass plate. The precision of re- 

 sponse varied correlatively with the force of gravity. In fact, the paral- 

 lelism was almost perfect. The question naturally rises, Is there a 

 similar parallelism between other stimuli and their responses ? 



A very little experimentation shows that the slug is extremely sensi- 

 tive to light. We have already seen how light may enter in to modify 

 the action of gravity. Casual observation shows that the response is in 

 most cases negative, — the animal moves away from the source of 

 light. Owing to its method of locomotion, the slug is easily experi- 

 mented on. It moves slowly and deliberately. In regard to its responses 

 to light, the following questions suggest themselves : (1) Are all indi- 

 viduals negatively phototactic to artificial light? (2) Does the precision 



