YELLOW-BROWN CELLS OF CONVOLUTA PARADOXA. 455 
be observed among the former, among the latter some 
few intact normal yellow-brown cells persisted. Since there 
is no apparent reason why the algal cells of animals supplied 
with proper food should suffer curtailment of supplies merely 
because they are placed in the dark it is reasonable to con- 
clude that, whatever may be the contribution toward the 
raw material of the food which the yellow-brown cells 
receive from the animal, they do not receive elaborated 
material from that source. The somewhat slower reduction 
of algal cells in the fed animals indicates that, in the presence 
of food derived from the outside, the yellow-brown cells are 
in some small measure spared. 
Taking all the facts together: from the behaviour of the ani- 
mals and aigal cells in filtered sea-water (light), and in filtered 
sea-water (dark), and in ordinary sea-water with weed (dark), 
it must be concluded that, in darkness, the algal cell failing 
to photosynthesise material for itself and unable to obtain 
supplies of elaborated food material from the animal, falls a 
victim to the digestive activity of the surrounding animal 
tissues. In its weakened state, brought about by starvation, 
it can no longer offer resistance to this digestive activity, 
whereas in its normal state it buys off this fate at the price of 
the tribute in kind, in the form of fat, which it pays continu- 
ously to the animal. 
Histological examination of the dark-kept animals supplies 
evidence that degeneration of the algal cells is not a mere 
decay within the animal body, but is the result of a process 
of digestion exerted’ on them by the animal tissues. The 
first sign of this digestive process consist in a reduction in 
size and a more spherical shape of the yellow-brown cells. 
The chloroplasts become smaller and rounder, though the 
nucleus and the clear anterior end of the algal cell may 
persist. Hach reduced algal cell may now be seen lying in a 
distinct digestive vacuole containing a pink fluid. The 
pigment of the chloroplasts is dissolved, and diffusing out of 
the cell may colour the vacuolar fluid brown. At this stage 
the chloroplasts are greenish, but later they become colour- 
