290 INVERTEBRATE PHYSIOLOGY 



it insures that a large number of males and females reach sexual maturity 

 and discharge their reproductive cells to produce such high concentrations 

 of them in the ocean as to provide a high likelihood that a sperm cell will 

 reach an egg cell. The precision of the synchrony of breeding is no better 

 illustrated than in the instance of the Atlantic fireworm (Huntsman, 

 1948). For three or four evenings each month during the summer at full 

 moon, and 55 minutes after sunset, these worms, luminescing brightly, 

 swarm in the waters about Bermuda. About a half an hour later only 

 occasional stragglers are left. The Palolo worm of the Pacific Ocean 

 swarms and breeds just as dawn is about to break at the third quarter of 

 the October and November moons (Huntsman, 1948). The seaweed 

 Dictyota releases its reproductive cells at full moon and high tide (Wil- 

 liams, 1905;Hoyt, 1927). 



An even more spectacular example of a precisely timed behavior pattern 

 is the breeding of a small pelagic fish, the grunion, of the Pacific coast of 

 the United States (Clark, 1925). On three or four nights when a spring 

 tide occurs in April through June, the male and female grunion swarm in 

 from the sea just as the tide has reached its highest point on the beaches ; 

 they are tossed by the waves onto the beaches where they quickly deposit 

 their eggs and sperm together in the sand. Then they flip back into the 

 water and are off to sea again. The fertilized eggs develop in the moist 

 sand and the young fish at the next spring tide, 15 days later, when the 

 spot is again submerged by waves, leave the nest for an open-sea existence. 



Another of the very interesting ways in which a biological clock may be 

 used by organisms is in their normal navigation. Birds use, at least to some 

 extent, in their homing or migration, the so-called "light-compass reac- 

 tion." In this, the sun is kept at a fixed angle with respect to the long axis 

 of the body. For short trips, the sun is sufficiently fixed so that this method 

 permits flight in a straight line. On longer trips, as in day-long flights of 

 birds in migration, the sun gradually moves through a considerable angle. 

 Kramer (1952) has recently found strong evidence for some clock in 

 the starling and pigeon which constantly corrects the orientation during 

 the day for the normal positional changes of the sun. He studied the 

 orientation of the birds in enclosures in which an artificial sun was held 

 constant. During the day, the general orientation of the birds systematically 

 shifted at a rate which one would have predicted on the basis of the birds' 

 continuously correcting for the normal rotation of the earth. Pardi and 

 Papi (1953) have shown that a small crustacean, Talitrus, navigates not 

 only by the sun, but also by the moon, in migrations to and from the 

 water's edge. These animals, too, appear to compensate continuously for 

 the changing positions of the sun and moon in the sky. 



Beling (1929), studying the time sense of bees, found that she could 



