466 FREDERICK KEEBLE. 
ingestion means only a somewhat increased death-rate. 
Hence, whilst to the infecting organism the effects of the 
relation are nil, these effects are permanently recorded in 
changed habit and modified development in the animal. 
Although to the species “infecting organism” it means go 
little, to the individual ingested algal cell incorporation in the 
tissue of the animal means much. The rapid growth and 
multiplication of the ingested algal cells indicate that the 
tissues of the animal offer a medium highly favourable to the 
development of those cells. So striking is this, that observers 
not infrequently infer from it that the alga is living parasiti- 
cally in the animal body. Though this has been shown in 
the preceding pages not to be the case, the problem remains: 
to what peculiar conditions obtaining in the tissues of the 
animal is due this luxuriant vegetative development of the 
algal cell ? 
It was suggested, when this same problem was under con- 
sideration in the case of C. roscoffensis, that the luxuriant 
growth of the ingested algal cells was due to the favourable 
position which they occupy with respect to nitrogen. It is 
known that the amount of nitrogen present in sea-water in a 
form available to plants for synthetic purposes is extremely 
low; so low that it is highly probable that the amount of 
nitrogen is the factor which limits the development of marine 
plants and animals. Thus, according to Johnstone (1907), 
who bases his calculations on Raben’s estimates, ‘‘the amount 
of nitrogen-compounds in Baltic and North Sea water may be 
taken as about ‘2 mgr. in a litre, or 2 parts in one million.” 
Whence it follows that the infecting organism, living free in 
the sea, must be hard put to it, in common with its com- 
petitors, to obtain sufficient nitrogen. Once in the body of 
C. paradoxa the state of affairs, so far as nitrogen is con- 
cerned, is changed. Since this animal possesses no excretory 
system, its waste nitrogen, stored within the. body, is at the 
disposal of the alga. As von Graff has suggested acutely 
(1891), the transverse bands of refractive substance, white by 
