YELLOW-BROWN CELLS OF CONVOLUTA PARADOXA. 457 
C. roscoffensis, which, as has been shown, is at birth 
altogether free from algal cells. That C. paradoxa hatches 
as an uninfected larva is easily shown ; indeed, whereas in 
C. roscoffensis the difficulty is to prevent infection by 
the algal cells, in C. paradoxa the difficulty is to induce it. 
If the egg-capsules of C. paradoxa are placed in ordi- 
nary sea-water, without any weed from the Paradoxa zone, 
the animals do not become infected. They feed on diatoms, 
etc., which may be present in the water, yet not only do they 
not increase in size, but rapidly become smaller, and fail 
altogether to show signs of development. Thus, a series of 
measurements of just-hatched animals gave length by breadth 
in divisions of ocular micrometer L. 2. 3 :—3500, 3600, 4500, 
5725, or in sq. mm.: 1°02, 1:04, 1:3, and 1:6; whereas a 
series of measurements of uninfected animals of various ages 
all put with water rich in diatoms and other food-material 
gave 1600, 1920, 2400, 2720, 3600, 4125, 4750, or insq. mm. : 
-5, 56, 69, °78, 1:05, 1:2, and 1:4. That is, whereas the 
average size of the just-hatched animal is about 1°2 sq. mm., 
the average size of 7 animals supplied with food but not with 
the infecting organism is ‘88 sq. mm.; and this in spite of 
the fact that the food-material (diatoms, etc.) is taken up 
readily enough. 
In order to induce the development of the yellow-brown 
cells in the larve it is not sufficient to cause them to hatch 
out in water from the Paradoxa zone. Weed from the zone 
must be provided, and the supplies of this weed replenished 
several times during the first few days after hatching. Even 
so, infection does not follow inevitably. In cases where, by 
the adoption of this procedure, infection ensued, it was found 
to occur very rapidly. 
For example, in one instance, animals were caught on 
August 12th; on the 14th eggs were found on the small 
pieces of weed left with them. The young, hatched on the 
16th, were put in a separate vessel with water and fresh weed 
from the Paradoxa zone. On the 17th the specimens examined 
showed no signs of yellow-brown cells. Another piece of 
