YELLOW-BROWN CELLS OF CONVOLUTA PARADOXA. 463 
stage, and that infection may arise from the former stage. 
The more normal, yellow-brown appearance presented by other 
algal cells which occur in just-infected animals likewise suggest 
that the alga may also be ingested in its holophytic stage. 
Like variations in appearance are presented by the infecting 
organism of C. roscoffensis, in its earliest stage in the body 
of that animal; and in this case it has been demonstrated 
that these variations in the earliest recognisable condition of 
the infecting organism are due to the fact that the latter may 
be ingested at almost any stage of its life-history, viz. as a 
green resting-spore, as a colourless resting-spore, as a green 
or colourless, non-motile daughter-cell, or as a four-flagellated, 
green, typically chlamydomonadine cell: all of which stages 
have been observed in the life history of the infecting 
organism of C. roscoffensis. Once within the body of 
C. paradoxa the yellow-brown cell develops rapidly, the 
daughter-cells to which it gives rise are sown about the body, 
and undergo further growth and division. By this means, and 
also possibly by formation of spores, the yellow-brown cells of 
the body, now to be numbered by hundreds, impart, together 
with the orange glands, the characteristic colour to the 
animal. 
To return to the question of the reappearance of normal 
yellow-brown cells in animals which, after long exposure to 
darkness in filtered sea-water, are brought into contact with 
weed of the Paradoxa zone (p. 454). 
The appearance presented by the new crop of algal cells, 
the smallness of their numbers, their large size and their 
resemblance to those of just-infected, larval animals, their 
occurrence side by side with colourless heaps of undigested 
granular remnants of the destroyed earlier crop of yellow- 
brown cells, all suggest that reinfection has taken place. And 
the fact that, when reinfection takes place, growth of the 
animal is resumed indicates how intimate has become this 
relation between alga and animal. 
(f) The Nature of the Infecting Alga.—Pending 
the discovery of the free stage in the life-history of the yellow- 
VOL. 52, PART 4.—NEW SERIES. 36 
