Corculum and Zooxantheliae — KAWAGUTI 
living state, without showing morphological 
fusion. 
The mantle of the upper side is pale and 
contains only a few zooxantheliae. The lower 
side, however, is dark brown in color due to 
the presence of the great accumulation of 
zooxantheliae. The difference is most strik- 
ing at the folding line of the mantle from 
upper to lower surfaces as is shown in 
Figure lb. The lower side is almost solidly 
dark but the upper side is only scattered with 
the zooxantheliae. 
Fig. 6. a, C. cardissa with shell removed. Seen 
from lower side. The many dark spots are colonies 
of zooxantheliae. (Natural size.) b, Body of C. 
cardissa as seen from upper side. (Natural size.) 
The mantle is extremely delicate and thin 
except at the edge. These relations may allow 
effective penetration of light to the gill fila- 
ments where the zooxantheliae are accumu- 
lated in great abundance. On the lower side, 
however, there is no need for the light to 
47 
penetrate through the mantle as there are no 
more zooxantheliae beneath the mantle. 
There are many pigment granules on the 
outer surface of the mantle. They are blue, 
reddish-purple, or red in color, with signif- 
icant fluorescence. They are a little larger than 
the dimensions of zooxantheliae. Whether 
these pigment granules may act as a filter for 
too strong light is not clear. 
Fig. 7. a, Gill filaments of Corculum in living 
state, showing a full accumulation of zooxanthel- 
iae in them. A series of cilia is seen on each gill 
filament, b, A part of folding zone of mantle 
showing the great accumulation of zooxantheliae 
in the lower portion (seen evenly dark) and the 
lesser scattering in the upper portion. 
