422 F. W. GAMBLE AND THKBEEICK KEEBLE. 



(c) Phototropism. — Convoluta is positively plioto- 

 tropic, but only under certain conditions. A sudden elevation 

 of light-intensity induces a negative phototropism. Back- 

 ground may inhibit phototropic response. 



At the moment of hatchings Convoluta is aphototropic. 

 The power of response to light develops rapidly^ appearing 

 a few hours after hatching. 



The anterior end only, of animals divided transversely, is 

 phototropic. 



The rays active in producing phototropism are the green. 

 The blue rays, which are active in inducing movement in 

 zoospores, in Euglena, and in plants generally, produce no 

 tropic effect in Convoluta. The red rays produce a feeble 

 negative tropism. 



The tonic influence of light is more important than the 

 tropic iufluence. To the former influence is due the 

 remarkable periodic '^ tidal " movement which, as we have 

 found, Convoluta performs. After a spell of insolation, 

 colonies sink below the surface, and after a certain sojourn 

 in darkness they return to the surface. These movements 

 synchronise with the covering and uncovering of the Con- 

 voluta zone by the tides. They take place in colonies 

 brought into the laboratory, but do not occur when such 

 colonies are kept in darkness. Convoluta, after a certain 

 spell of illumination, passes into a condition of light-rigor. 

 In this condition it is singularly inert and susceptible to 

 mechanical injury. 



(d) llheotropi&m. — Convoluta reacts to changes in rate 

 of flow of water. In a moderate stream it tends to move up 

 stream. Under the stimulus of a suddenly increased flow it 

 sticks to the ground; when this is impossible it contracts 

 itself and is carried down stream. 



4. Ecology. 

 A. Previous Observations. — Geddes and von Graff 

 have recorded the distribution of Convoluta along the 

 shores of the lie do Batz, lloscoff. They describe the rapid 



