32O S. J. HOLMES. 



much as higher organisms collect in situations where it is neither 

 too warm nor too cold. In the experiments of Oltmanns we 

 certainly cannot assume that the factor of the direction of the 

 rays has been excluded. Owing to the scattering of the rays 

 entering perpendicularly through the sides the light end of the 

 vessel, in such an apparatus as Oltmanns employed, must act 

 practically as an independent source of light. If, therefore, Vol- 

 vox is positively phototactic in weak light and negatively so in 

 strong, we may readily understand why collections are formed 

 in regions of a certain intensity of illumination in accordance 

 with the theory that the direction of movements is determined 

 by the direction of the rays. 



As an opportunity presented itself this last fall of procuring 

 Volvox easily and in large numbers I endeavored to work out 

 some points in the phototaxis of this organism a little more in 

 detail, and, if orientation to the direction of the rays should be 

 found to occur, as seemed probable from the statement of pre- 

 vious observers, to ascertain the method by which the orienting 

 response is brought about. It is easy to determine that Vofao.r 

 orients itself, and that very accurately, to the direction of the 

 rays of light. If specimens of Volvox are taken into a dark 

 room and exposed to the light from an arc lamp they travel 

 towards the light in almost a straight course, swerving remarkably 

 little to the one side or the other. They will often travel a foot 

 without deviating as much as a quarter of an inch from a perfectly 

 straight course. If the position of the light is changed during 

 their progress they soon re-orient themselves and travel straight 

 onwards as before. If the light is placed at the other end of the 

 vessel they turn about and come back to where they started. 

 The shape of the Volvox is not quite spherical but slightly 

 elongated, forming a prolate spheroid, and when swimming 

 through the water the organism rotates on its long axis. As is 

 well known the anterior end of Volvox may be distinguished by 

 the fact that it is usually free from daughter colonies, and Ryder 

 has pointed out that the red ocelli of the zooids are much larger 

 at this end than elsewhere and diminish gradually in size towards 

 the posterior end of the body. While swimming towards the 

 light the largest ocelli are always directed towards the region of 

 greatest illumination. 



