46 
PACIFIC SCIENCE, Voi. IV, January, 1950 
culated in column 9. These data also bear 
out the same fact. 
In a first glance at the shell, the lower 
side seems to be thicker than the upper side. 
But actually they are almost of the same thick 
ness. This apparent difference is a result of 
the greater transparency of the upper side. 
On this side, there is a beautiful pattern which 
is shown in Figure 5. The pattern is formed 
by the mosaic arrangement of transparent 
mass and nontransparent whitish mass. On 
the lower side the transparent portions are 
scattered, rarely in rather large masses, and 
fade into whitish ground. Transparency of 
the shell is intensified if the shell is immersed 
in water because of elimination of reflected 
light from the uneven surface. 
Fig. 5. Enlarged view of pattern on the shell 
of Corculum. The pattern is formed by the mo- 
saic arrangement of transparent mass and non- 
transparent whitish mass. 
LARVAL DEVELOPMENT 
Corculum is a hermaphroditic animal. 
Development is indirect and it is remarkably 
rapid. Spawning was once observed on Feb- 
ruary 29, 1940, in the laboratory. A speci- 
men collected on the previous day and cul- 
tured in a glass basin extruded eggs at 11 
o’clock in the morning; by the end of 2 hours 
they had reached the two-cell stage. In the 
evening they had become veligers and began 
to move. The next morning, all of them had 
grown to the two-valved stage and were 
swimming at the surface of the culture 
medium. They grew into the usual type of 
bivalve shells by the next evening, and set- 
tled to the bottom. Up to this time, there 
was no indication of the heart shape which 
is characteristic of this animal. There are no 
zooxanthellae in any tissue at this time. In 
what stage they attain the association with 
zooxanthellae is not yet known. 
ASSOCIATION WITH ZOOXANTHELLAE 
Gills 
The normal position of the internal organs 
is somewhat displaced in accordance with the 
deformation of the shells. The organs are 
shown in Figures 6a and 6b. The gills and 
mantles are the most interesting. The gills 
are situated at both sides dorsally. They are 
short and narrow in figures, even in the photo- 
graph in the living state (Fig. 6). In the 
active state, however, they expand fully, just 
under the mantle, almost filling the whole 
mantle cavity. The length of the gill fila- 
ments of the inner lamella is much greater 
than that of the outer lamella, especially at 
the middle portion. This serves to fill the 
mantle cavity with gill filaments in the active 
state. 
The gill is dark brown in the living ani 
mal. When it is examined under a micro 
scope many zooxanthellae are found to be 
present in the filaments (Fig. la). The zoo- 
xanthellae do not seem to be enclosed in the 
host’s cells. 
Mantle 
The mantle shows similar modifications in 
its structure and relative position. It forms a 
thin and heart-shaped mantle cavity. The 
mantle edges are fused at the middle of the 
upper side into an exhalant siphon; more ven 
trally they form an inhalant siphon, in the 
