220 
PACIFIC SCIENCE, VoL XI, April, 1957 
ready possessed fertilization membranes. As 
stated above, sperm were observed embedded 
in the space within the fertilization mem- 
brane. Cleavage proceeded very slowly through 
the eight-cell stage. The cleavage pattern was 
spiral and similar to that described by E. B. 
Wilson (1892) for Neanthes succinea and Platy- 
nereis dumerilii . However, there was no further 
cleavage of the four yolky macromeres be- 
yond this stage of development. These yolk 
cells will eventually become enclosed within 
the digestive tract and will be utilized for 
food (Figs. 2 to 7). These four cells can be 
seen until just prior to feeding. 
Cleavage beyond the eight-cell stage was 
limited to the animal pole (Fig. 2, arrow). 
These cells come to surround the four macro- 
meres by epibolic growth. The embryo was 
never ciliated, which is unique among the 
nereids. 
The developing egg remained spherical un- 
til the seventh day when a slight elongation 
of the embryo was noted (Fig. 3). Longi- 
tudinal muscular movement appeared at this 
time. The course of growth from the elonga- 
tion of the embryo through sexual maturity 
has been summarized in Table 2. Formation 
Fig. 2. Photomicrograph of developing embryos of 
about 18 hours in age showing the four macromeres. 
Arrow indicates site of cleaving micromeres. Diameter, 
520 ix. 
- ijJljl ,|l|| 
Fig. 3. Photomicrograph of an elongating embryo 
of seven days in age encased in egg capsule. Length, 
520 ix. 
of the three larval segments occurred at eight 
days. The anal cirri appeared as knobs at this 
time, but there were no indications of cephalic 
appendages (Fig. 4). Shortly thereafter the 
larvae hatched from the egg capsule. 
During the ninth day the fourth larval seti- 
gerous segment, the first pair of peristomial 
tentacles, and the tentacles appeared. In addi- 
tion, as many as four presumptive segments 
were often discernible as simple protuber- 
ances (Figs. 5 to 7). The four yolk-cells were 
still present, and because of their large size, 
they gave a humpback appearance to the 
larvae (Fig. 5). New segments were added 
rapidly until the animal had developed about 
16 segments at 16 days (Figs. 5 to 8). The 
young worms, which have been crawling ac- 
tively within the parent tube, begin to aban- 
don the tube in the next few days. The larvae 
crawl about the petri dish, construct mucoid 
tubes, and begin to feed upon Enteromorpha 
sp. by 21 days. There is no true planktonic 
stage in N. caudata. However, if the young 
animals are disturbed, they abandon their tube 
and either crawl or swim away. This behavior 
may play an important role in the distribution 
of N. caudata , particularly within a bay or 
