128 PLANT-ANIMALS [CH. 



in plenty, lying and dividing on its egg-capsule and 

 on any other organic debris. Should it escape in- 

 fection a rare contingency C. roscoffensis may, as 

 we have learned, return to the egg-capsule and thus 

 incur it. Or, becoming positively phototropic, the 

 larva moves up to the light. There, at the upper 

 edges of the water-films, it finds assembled the green, 

 flagellated organism. Beset in darkness and in light 

 by the infecting organism, swallowing eagerly all the 

 minute particles that come its way, C. roscoffensis 

 cannot escape its destiny. A colourless or green 

 cell is taken into the body and the plant-animal is 

 formed. 



So pleomorphic is the infecting organism that it 

 occurs in yet other forms beside those described 

 already. It may give rise by repeated divisions to 

 groups of rounded cells lying together in a colony. 

 Such a colonial form is known as a palmella stage 

 and occurs in the life histories of various green 

 algae. It is remarkable that a colourless palmella 

 form also exists. At any moment, a green member 

 of the palmella may slip its mucilaginous coat and 

 appear as a flagellated, active cell. 



As to the name and position in the plant kingdom 

 of the infecting organism we need say but little. Its 

 characteristics are those of a group of primitive, 

 green algse know as the Chlamydomonadinese. Like 

 the infecting organism, the members of this group 



