382 
dorsally. The furcal setae passed back to the 
region between or just posterior to the eyes 
(hence over the region of the anterior part of 
the yolk mass). 
Because of the proximity of the habitat to 
the sea, it was uncertain whether hatching oc- 
curred in fresh water or in sea water. Some of 
the eggs were placed in sea water, but after a 
couple of hours they showed distinct signs of 
degeneration. Controls in fresh water remained 
normal. 
Hatching commenced on the sixth day after 
capture of the animal. The size of the eggs had 
increased slightly to an average of 520/a X 
310 /a. At this time some more eggs were placed 
in sea water; they did not hatch successfully, 
whereas nearly all of the controls in fresh water 
did so. 
Hatching was observed many times. Two 
membranes surrounded the egg (a third mem- 
brane, as described by Burkenroad, 1947, was 
not observed). Before hatching, the embryo lay 
snug within the two membranes, but it was able 
to move around in such a fashion that it was 
clear the embryo itself was not exerting pressure 
on the outer membrane. 
The first event of hatching was the sudden 
bursting of the outer membrane, always at the 
cephalic end of the embryo. Immediately after- 
wards the inner contents, surrounded by a sec- 
ond membrane, almost explosively increased in 
size, showing that they were under considerable 
internal pressure. At this time the dimensions 
of the embryo and its membrane were about 
600/a X 320/a. Soon there was a rather rapid 
enlargement to 650/a X 330/a. As this swelling 
occurred, the outer membrane slipped to the 
posterior end of the egg and was sloughed off. 
At first after the bursting of the outer mem- 
brane the inner membrane lay tightly appressed 
against the body of the embryo, but later a fairly 
large space developed between the living animal 
and the nonliving membrane. At the same time 
the folds of the animal loosened up somewhat 
within the membrane, so that spaces appeared 
ventrally between the thorax and the abdomen. 
Then the young animal commenced to strug- 
gle, primarily by movements of the abdomen. 
Eventually the inner membrane broke over the 
end of the telson, hence over the head (no egg 
PACIFIC SCIENCE, Vol. XVIII, October 1964 
burster was evident on the telson), and the 
animal straightened out. In this manner the 
membrane was torn still more. Then a single 
vigorous flip was sufficient to liberate the zoea 
from the remains of the egg membrane. 
Five specimens were timed in their emer- 
gence. They took from 2 min 15 sec to 17 min 
5 sec (average of 6 min 35 sec) from the first 
bursting of the outer membrane to the last flip 
from the inner membrane. 
Discussion 
Hatching in decapod crustaceans has seldom 
been described heretofore. Among publications 
touching on the process, Davis (1959) sketchily 
observed hatching in the Ohio fresh-water 
shrimp, Palaemonetes kadiakensis, and stated 
that osmotic forces were involved. R. L. Robert- 
son, writing in Truitt ( 1941 : 10) , gave a figure 
showing hatching of an egg of the crab, Calli- 
nectes sapidus. The prezoea is shown escaping 
backwards, with the rear of the head coming 
out first; only an outer membrane is shown. 
Churchill (1917-1918) stated concerning the 
same species: ". . . the shell of the egg split into 
two parts, the young crab emerged and, after 
freeing itself from a thin membrane which cov- 
ered it, swam away.” Gray (1942) briefly de- 
scribed hatching of the fiddler crab, Uca minax. 
She intimated it was caused entirely through the 
vigorous mechanical action of the crab; only 
one egg membrane was mentioned. Andrews 
(1904, 1907) studied hatching in the crayfishes 
Astacus leniusculus and Cambarus affinis, but 
merely stated that: "In hatching, the egg capsule 
burst open over the back of the embryo, and . . . 
then the embryo slowly glided out backward.” 
According to his observations the embryo at 
hatching was very inactive and helpless, unable 
to use its limbs. More recently, Burkenroad 
(1947) described hatching in the marine shrimp 
Palaemonetes vulgaris and in its close relative 
from fresh water, P. exilipes. In the marine 
species, the outer two membranes were burst 
by swelling of the young within, and the em- 
bryo, enclosed in the third membrane, emerged 
passively. It then tore its way from the third 
membrane by active extension of the pleon. In 
the fresh-water form, on the other hand, the 
outer membranes were split through the pressure 
