236 Journal of Comparative Neurology and Psychology. 



ellipses within the illuminated regions, for the black regions alternately attract and 

 repel them, as is obvious from the previous statement. As the author points out, 

 the devious courses of the molluscs, often interpreted as food-seeking or other 

 forms of voluntary movements, are describable in terms of just such relations of 

 forces as those studied in this memoir. It is easy enough so to alter the relations 

 of light and gravity, from moment to moment, that an animal under their 

 influence moves in an irregular course, as if seeking something. 



Of the variations in response to light in Littorina, there are irregular changes 

 due to variations in the illumination or other factors which condition the influence 

 of light, and regular, rhythmic changes due to the tidal conditions of the sea 

 (hydration). Hohn has discovered that certain fluctuations in the light reactions 

 of the three species of Littorina studied, are synchronous with the movements of 

 the sea, while the amplitude of the fluctuations is proportional to the amount of 

 hydration. Furthermore, there are two kinds of fluctuations in the response "to 

 light; one of great magnitude corresponding to the period of the great tides (about 

 14 days), the other of slight magnitude, corresponding to the daily tides (about 

 13 hours). The animals continue to exhibit rhythmic fluctuations in their 

 reactions to light for months after being subjected to conditions of life in an 

 aquarium, but the amount of the change progressively decreases. 



As is stated early in the memoir, the three species of Littorina used, live at 

 different levels on the shore. Those which live at a high level (L. rudis), and 

 which therefore undergo prolonged and intense desiccation, habitually move, ac- 

 cording to the direction of the luminous field, in the negative sense (negative 

 phototaxis). Those of the low levels (L. obtusata,) which undergo only very 

 brief and slight desiccation, habitually move in the positive sense. The change in 

 the sense of the phototactic reaction corresponds, in the first case, to the period 

 when the tide is highest, in the second case to the period of low water. The 

 species of the middle level (L. littorea) show changes in reaction the more pro- 

 nounced the greater the amount of desiccation. In all cases, v\ hen the animals are 

 in an aquarium, toward the time of low water, the time at which desiccation 

 occurs in nature, the direction of the luminous field undergoes a negative change 

 that is to say, it inclines toward the black screen (No. 28 p. 54). 



Aside from the importance of the many new facts which Bohn has discovered, 

 it is evident that his work is of great value, in that it emphasizes strongly the 

 necessity, in the study of the tropisms, of considering many factors rather than 

 one alone, and of past influences as well as present influences. The behavior of 

 Littorina in response to light can be understood only after a study of the habits 

 and habitat of the various species, and of the influence of tides, moisture, 

 temperature, gravity, etc. 



The investigation of the reactions of Hedista has demonstrated that they 

 exhibit oscillations in their responses to light much as do the Littorina. The 

 direction of movement is determined in part by the relative intensities of the 

 illumination of the two eyes. "When the Hedista have undergone desiccation, 

 light brings about, through the eye and the nerves, muscular movements, and it 

 results: (l) That the worms come to rest in the shadow (negative phototropism, 

 or in Bohn's terminology, phototactisme negatif); (2) that they direct them- 

 selves toward the dark surfaces, the path therefore curving toward the side of 



