YELLOW-BROWN CELLS OF CONVOLUTA PARADOXA. 465 
tion as a chloroplast-containing cell of one of the higher 
plants stands to the colourless cells of that plant. The 
parallel is even closer than might seem from the foregoing 
for, as will be shown immediately, just as the chloroplast- 
containing cell of a plant receives unelaborated food-material 
from the colourless cells and, in return, provides them with 
organic food substances, so the yellow-brown cell receives 
from the cells of the animal the raw material from which it 
elaborates organic food substances, and in exchange passes on 
organic food substances to the animal tissues. As was pointed 
out in a previous paper (Keeble and Gamble, 1907), a pre- 
cisely similar relation holds between green Chlamydomona- 
dine cell and C. roscoffensis. Viewed from the stand- 
points of the animals, C. roscoffensis and C. paradoxa, 
the relations between them and their respective algal cells 
admit of accurate definition. C. roscoffensis and C. 
paradoxa are obligate parasites. In the absence of their 
respective infecting organisms these species would become 
extinct. 
But if the standpoint is shifted so that the relation 1s 
regarded from the point of view, so to speak, of the yellow- 
brown cell, the definition of this relationship will vary accord- 
ing as attention is directed to the individual yellow-brown 
cell or to the species of which that cell is a member. To the 
species, the relation is a meaningless episode; to the indi- 
vidual, ingested, yellow-brown cell, it is a great event. All 
_ that ingestion means to the species is that a certain, probably 
small, proportion of its members meet their fate in the body 
of C. paradoxa. It is only another hazard in the struggle 
-for existence. The algal cells which are incorporated into 
the tissues of C. paradoxa never escape and so never im- 
press the species with the consequences of that event; any 
modification which the algal cells may undergo as a result of 
their sojourn in the animal body can leave no mark on the 
species. 
To the species C. paradoxa, infection or non-infection are 
matters of life or death; to the species “infecting organism,” 
