THE GREEN CELLS OF CONVOLUTA ROSOOFFENSTS. 187 



Among the Euglense and also among Diatoms it is known 

 that green (oi* brown) cells may lose their pigments, and, 

 assuming a coloui-less state, exchange a holophytic for a 

 saprophytic existence. 



From the fact that when infection of Convoluta occurs 

 normally from sea-water the earliest recognisable stage of the 

 infecting organism in the body of the animal is a coloni'less 

 or faintly yellow cell it -would appear that the colourless 

 phase of the alga described above represents a perfectly 

 normal stage of its life-history. 



The suggestion ma}^ be hazarded that the formation of 

 colourless cells capable of a saprophytic mode of life is an 

 adjustment which widens enormously the range of distribution 

 of the alga. The green, active cells swarm toward the light, 

 and so have their distribution determined by the light factor 

 of their environment. The colourless forms, living sapro- 

 phytically, increase by division even in darkness. Thus the 

 alga may be enabled to live, not only in the surface waters, 

 but also below the surface of the sand wherever organic 

 debris provides material for its support. Such a divided 

 habit would undoubtedly be of the utmost service to the alga, 

 for, attached to the mucilaginous film which surrounds the 

 body of Convoluta or to the gelatinous egg-capsule, it under- 

 goes immersion deep in the sand at every tide. As each tide 

 arrives at the patch of sand covered by the vast colonies of 

 Convoluta these latter descend, as we have described in an 

 earlier paper, out of reach of the disturbance caused by the 

 moving water. The eg'gs of Convolutn, moreover are laid, 

 not on, but beneath the surface of the sand, so that an 

 organism which adjusted its habits so as to become an associate 

 of Convoluta would be compelled to pass many hours of the 

 day, and not infrequently, when attached to the egg-capsules, 

 many days, in darkness. This it could support only if it were 

 capable of a saprophytic habit. 



That the infecting alga is capable of such a habit cannot 

 be doubted, for, apart from the colourless stage being regarded 

 as evidence, the active green cells themselves give corrobora- 



