222 
PACIFIC SCIENCE, Vol. XI, April, 1957 
Fig. 6. Photomicrograph of seven-setigerous seg- 
mented larva 11 days old. Appendages as in Figure 5. 
Note the humpback appearance of the animals in 
Figures 5 and 6. Length, 1.1 mm. 
material the 20-segmented stage was attained 
about eight days before the Cherbourg popu- 
lation. 
Later Stages of Growth 
Growth in the later stages of development 
in the nereids has been measured in three 
different ways. The use of segment number 
per unit of time has been employed most 
frequently (Banse, 1954; Bogucki, 1954; 
Dales, 1950; Hauenschild, 1951; Hemple- 
mann, 1911; Herpin, 1926; Rullier, 1954; and 
D. P. Wilson, 1932). Body length was utilized 
by Bogucki (1954), Dales (1951), and Smidt 
(1951). The weight of the animal was deter- 
mined periodically and correlated with meta- 
morphosis and the appearance of genital 
products in Nereis gruhei by the author (1954). 
With the former two methods, a sigmoid 
curve for growth was obtained. Similar results 
were attained for N. gruhei when the number 
of segments per unit time was plotted (Fig. 
8) using the data given by Reish (1954). 
Growth in Neanthes caudata was followed 
by determining the number of segments pres- 
ent per unit time; the results have been sum- 
marized in Table 1 and plotted in Figure 8. 
The data are based on the growth of the 
offspring of one mating. Observations upon 
living specimens are difficult to make since 
they move about continually within their 
tube. The activity of these animals was slowed 
by placing the petri dish within an ice bath. 
It is readily observable that growth in N. 
caudata differs from Nereis gruhei . The appear- 
ance of the three larval setigerous segments 
lags in N. caudata , but the attainment of 
additional segments in N. caudata soon sur- 
passes Nereis gruhei. However, a plateau is 
reached at the time the larvae of N. caudata 
leave the parent tube and commence feeding 
at the 17- to 19-setigerous segment stage 
(Fig. 8; Table 1). Shortly thereafter a second 
growth spurt occurs which continued until 
the eggs were first discernible in the females 
at around the 5 5 -segment stage. A few addi- 
tional segments were added during the next 
10 to 15 days. The maximum number of 
Fig. 7. Photomicrograph of eight-setigerous seg- 
mented larva 12 days old. Appendages as in Figure 5. 
Length, 1.2 mm. 
