PHYLUM ANNELIDA 279 



Figure 15.8. Two earthworms copulating. (Photograph of living animals made at 

 night, courtesy General Biological Supply. Chicago, 111.) 



and the openings in it constrict to produce a spindle-shaped capsule. 

 The eggs develop into tiny worms which later emerge from the cocoon. 



126. Reproductive Periodicity and Palolo Worms 



External fertilization like that of the polychaetes is Usually accom- 

 panied by a coordinating behavioral mechanism that will ensure fer- 

 tilization. Many such organisms respond to rhythms in the environment 

 to achieve this coordination. In the oceans three such rhythms are domi- 

 nant. Seasonal cycles produce variations in temperature, length of day 

 and food. Lunar cycles produce variations in the height of tides, strength 

 of currents, the relation bet^\'een tide and the hour of the day, and the 

 amount of night light. Diurnal cycles produce the obvious great varia- 

 tion in light from day to night. Several species of Nereis use all three 

 of these rhythms to achieve reproductive periodicity. 



In a common Atlantic nereid (Platynereis) the adults become sex- 

 ually mature only in the summer months, some individuals breeding 

 several times in one season. During this season they reach sexual ma- 

 turity only during the second and third weeks after the new moon, 

 possibly because during this time the moon is bright and shines much 

 of the night, providing the dim light in which nereids will feed. The 

 actual moment of breeding depends upon the diurnal cycle. They will 

 breed only after dark, but only if the moon is not yet risen. Thus, worms 



