THE BIONOMICS OF CONVOLUTA ROSCOFrENSIS. 421 



The evidence, however, points most strongly to infection. 

 The infecting organism is a colourless cell. Infection often 

 takes place from the capsule. The situation and colourless- 

 ness of the infecting cell suggest the view that this cell is a 

 saprophytic stage in the life-history of the green cell. Such 

 saprophytic colourless stages are well known in green algte, 

 diatoms, and Flagellates. 



The colourless cells are taken up in company with other 

 organisms. A pure culture is made from this mixed infection 

 in the gut of the animal. There the cells become green, 

 divide, and, continued in wandering cells, are carried to their 

 final station in the periphery of the body. 



We regard the presence of the green cells as due to a 

 peculiar and special case of phagocytosis in which neither 

 green cell nor wandering cell is destroyed. 



The relation between animal and green cell is a complex 

 one, and cannot be described as symbiotic. The green cell 

 once in the body of the animal probably never escapes ; 

 either it is digested or it dies when the animal dies (p. 375). 



3. Tropisms. 



A. Previous Observations. — Greddes (1879) first recorded 

 positive phototropism (phototactism). Von Graff and Haber- 

 landt(1891) observed negative geotropism in stillness, positive 

 geotropism in the presence of vibrations. 



B. Observations of the Authors. — (a) Geotropism. 

 — The observations cited above were confiraied. The young 

 at hatching react to gravity in the same way as adults. In 

 the absence of the otolith, geotropism does not occur. 



The reflex mechanism involves the anterior end of the 

 body. The hinder end of animals divided transversely does 

 not react to gravity. 



(6) Thermotropism. — Convoluta is attuned to a high 

 light-intensity. It is athermotropic at ordinary temperatures. 

 Just below the lethal point (38° C.) it is negatively thermo- 

 tropic. The response, however, is singularly and often fatally 

 imperfect. 



