252 MR. M. C. POTTER ON AN 
removing a patch and cutting sections in a direction perpendicular 
to the animal’s back, it is found to be composed of numerous 
rather large cells arranged very close together and generally square 
in shape. Some of these cells are exposed to the action of the 
water, and form a plate of cells a few layers thick, closely applied 
to the tortoise-shell, and others are found as wedge-shaped masses 
which penetrate into the shell. The cells of the plate whose free 
surface is exposed to the action of the water continually divide 
in directions perpendicular and parallel to the surface of the 
tortoise-shell ; the outermost layer continually forms zoospores, 
and so prevents the plate from becoming more than a few cells 
thick. The cells next to the tortoise-shell are closely adpressed to 
it, and individually have the power of penetrating into any crack 
of the tortoise-shell which may present itself to them. When 
an algal cell meets with a crack, it strives to penetrate into it, 
in doing which it opens the crack more and more and so pene- 
trates further in, and as it does so it divides first by a plane 
perpendicular to its direction of growth and then by planes 
perpendicular and parallel to this direction, and so forms 
wedge-shaped masses of cells, as at a in figs. 2 and 3, which grow 
and penetrate in any direction in which they can force open the 
crack. Sections cut parallel to the animal’s back show on the 
outside masses of algal cells where the thick ends of wedges are 
eut through (a, fig. 3), and towards the centre sections of 
thinner parts of wedges (b and c, fig. 3). Thus a section through 
an algal patch similar to fig. 2, parallel to the animal's back, 
would be represented by a drawing resembling fig. 3. 
If sections of the tortoise-shell and alga are allowed to remain 
in water for some few days, it is found that not only does the alga 
remain alive but continues to grow healthily. The cells exposed 
to the water continually form zoospores, while the layers in con- 
tact with the tortoise-shell, since the surrounding pressure is 
removed, tend to grow out into filaments (a and 5, fig. 4). 
These filaments can be formed by any cells touching the tortoise- 
shell; they have a very irregular shape, and the chlorophyll is 
always situated at the growing end. 
As before mentioned, the alga is reproduced by means of zoo- 
spores which&re formed from the outermost layers of cells. The 
cell about to form a zoosporangium becomes flask-shaped, a kind 
of neck being formed, the contents of the cell dividing into a 
considerable number of zoospores, which have the usual pyriform 
