Hawaiian Marine Gastropods — OSTERGAARD 
Eggs were deposited in the laboratory by 
one of these animals on September 3, 1921. 
Since that time they have often been obtained 
from other individuals. The egg structure was 
a lemon-yellow, tough, gelatinous mass with 
irregular outlines, pasted flatly onto any sur- 
face. The ova were closely packed through- 
out the whole structure and each was enclosed 
within a double capsule. The diameter of the 
ovum was about 0.08 mm. The lesser diam- 
eter of the inner capsule, which was of a 
regular oval or oblong form, was 0.1 4 mm., 
its greater diameter being 0.22 mm. The 
lesser diameter of the outer capsule, which 
was of an irregular oval or somewhat baggy 
shape, was 0.2 mm.; its greater diameter was 
about 0.28 mm. 
The relations of the capsules to one an- 
other appeared to be as follows: from each 
end of the inner capsule sprang a slender tube 
or thread which united with another inner 
capsule, penetrating the outer capsule in do- 
ing so, forming thus a continuous, closely 
united string of egg capsules thrown promis- 
cuously together (Fig. 42a, b ) . 
Cleavage of the zygotes commenced the 
same day they were laid, the 4-cell stage be- 
ing reached during the afternoon. The first 
two divisions were equal and the third re- 
sulted in four micromeres at the animal pole 
of the macromeres; hence it was typical of 
gastropod cleavage (Fig. 42c-/). 
A many-celled, or morula, stage was reached 
after 24 hours. The four undivided macro- 
meres showed up distinctly at the vegetal 
pole, their nuclei appearing as clear globules. 
From the animal pole numerous micromeres 
extended beyond the equator. Soon a con- 
traction of each of the four undivided macro- 
meres took place, rounding off their former 
angular outlines and leaving an opening be- 
tween them (the gastropore?) at the vegetal 
pole. Cleavage of the macromeres seemed 
greatly retarded, probably due to the obstruc- 
tion of the yolk. This is typical epibolic 
gastrulation (Fig. 42 g, h ) . 
113 
The extremely slow division of the macro- 
meres, or endodermal cells, remained a char- 
acteristic feature of the development of this 
mollusk. Even when the veliger stage was 
reached, these cells appeared as a large clus- 
ter, each with a distinct, rounded outline and 
a clear central nucleus (Fig. 42/). 
The free-swimming stage was attained 
after 9 days of incubation, when a veliger 
larva appeared within an elongate, clear, 
colorless shell which was nearly bilaterally 
symmetrical. The foot bore a broad, ante- 
riorly rounded operculum. Eyes, as well as 
otocysts, were present. The endodermal cells, 
still large and conspicuous, were of a bright 
yellow or brassy color (Fig. 42 j, k, /). 
CONCLUSIONS 
Pelagic Stage and Distribution 
Of all the species studied — those included 
in this paper and others to be reported upon 
later — 4 1 attained the veliger stage. Of these, 
40 were found to have a pelagic stage, thus 
giving the high figure of 97.5 per cent pelagic 
or free-swimming larvae. 
This seems remarkable in view of the fact 
that Thorson (1940) reports 75 per cent 
from the Iranian Gulf as the highest percent- 
age in a list prepared by him, while next high- 
est he quotes Risbec from New Caledonia 
with 57 per cent. He shows a general increase 
in pelagic larvae from the colder to the 
warmer waters, beginning with none in East 
Greenland and ending with 75 per cent for 
the Iranian Gulf. 
It appears that temperature is of much im- 
portance in determining the pelagic stage, 
but there seem to be other controlling factors 
also. When the isolation of the Hawaiian 
Islands caused by their vast ocean barriers is 
considered, it may be inferred that its mol- 
luscan population must have depended upon 
a pelagic or free-swimming stage of sufficient 
duration to enable the larvae to be carried by 
currents from islands in the Pacific support- 
