YELLOW-BROWN CELLS OF CONVOLUTA PARADOXA. 471 
step further in the former species. For, unlike C. paradoxa 
which continues all its life to feed voraciously, C. roscof- 
fensis ceases very early to ingest solid food. 
It is also noteworthy that the interpretation of the relation 
between animal and algal cell here offered throws hight on 
certain facts concerning the distribution of algal cells in the 
bodies of various marine animals. The analyses of Natterer 
and Raben have demonstrated that the amount of combined 
nitrogen present in sea-water is less during the warm months, 
e.g. August, than during the cold months of the year, and 
that it is less in the warmer seas (Mediterranean) than in the 
colder seas (Baltic and North Sea) (Johnstone, 1907). 
Now, as stated in the paper on C. roscoffensis, certain 
animals possess green or brown algal cells in one part of their 
range of distribution, but lack them in other stations. ‘Thus 
Noctiluca is colourless in the North Atlantic, and green in 
the Indian Ocean. British Aleyonium have no Zoochlorelle, 
whereas the closely allied A. ceylonicum possesses them. 
It seems probable, indeed, that the maximum development 
of these associations occurs in the warmer seas.” It would 
seem also probable that this parallelism between presence of 
algal cells and poverty of nitrogen is no coincidence, but that 
the former is causally connected with the latter. 
The paleness of the colour of the infecting organism in its 
earliest stage of existence within the body of C. paradoxa 
suggests that this organism has in its free stage both holo- 
phytic and saprophytic phases. The similar behaviour of the 
infecting organism of C. roscoffensis has also been estab- 
lished. In its earliest stage within the body of the animal it 
- may be altogether colourless. Such alternative phases, holo- 
phytic and saprophytic, pigmented or colourless, are known 
to occur in the life-histories of various of the lower. orga- 
nisms; for example, Diatoms (Nitzschia), certain Chlamydo- 
monadinez and Flagellates (Huglena). It is stated that the 
colourless phase may be induced by increasing the amount of 
soluble carbohydrate in the culture medium, or, in Nitzschia, 
according to Karsten, by augmenting the supply of organic 
