392 ANNUAL- EEPORT SMITHSONIAN INSTITUTION, 1920. 



place in the evenings before nightfall from 6.30 to 7 p. m. After 

 dark the flight was reduced to practically nothing. During the 

 period of flight, the observers were bitten continuously. After the 

 flight ceased, the observers were bitten only once or twice in an hour's 

 time. A return flight began at 6 a. m. and took place with extraordi- 

 nary rapidity. As daylight became stronger, the speed of the return- 

 ing Anopheles increased. The termination of both forward and re- 

 turn flights was remarkably abrupt. One observer said the flight 

 stopped with almost mechanical precision when there was too much 

 daylight or too much darkness." 



Willey G records an interesting example of periodic habit occurring 

 among crows and so-called "flying foxes" (really fruit-eating bats) 

 on the coast of Cejdon. At one place, a small lighthouse islet off the 

 coast of Ceylon, they congregate in the palm trees alternately by 

 night and by day. "At sundown," says Willey, " the passage of im- 

 mense flocks of crows and flying foxes in opposite directions across 

 the strait which divides the island from the mainland can be wit- 

 nessed, the former bound for the island to rest for the night, the 

 latter speeding their way to the mainland intent upon their nocturnal 

 forage. * * * The reverse passage — namely, the matutinal 

 flight — takes place toward sunrise, the bats returning from the main- 

 land to rest for the day suspended in rows from the midribs of the 

 palm leaves, the crows crossing over on their daily quest for gar- 

 bage." As a result of these markedly periodic habits, the two classes 

 of animals are able to make their homes in the same trees without 

 in the slightest degree interfering with each other. 



The effect of external periodicities on the organism and its behavior 

 is nowhere better seen than on the seashore. The case of Convoluta 

 roscofensis is perhaps too familiar to need much description. Con- 

 voluta roscoffensis is a minute, elongated flatworm covered with 

 cilia, and containing green algal cells living with it symbiotically. 

 Its habitat is a narrow strip of sandy beach on the coasts of Nor- 

 mandy and Brittany situated at the level reached by high water at 

 the slackest of neap-tides. Though of very small size, the worms 

 occur in such enormous numbers as to form at low tide great patches 

 of green scum. As the tide laps the edges of the colony the green 

 patches disappear, the worms remaining beneath the surface till the 

 . next ebb tide. Twice during 24 hours the zone occupied by the colo- 

 nies is submerged and the animals live in darkness underground, 

 and twice the zone is uncovered and the animals rise to the surface. 

 The burrowing reaction is due to the necessity of avoiding extermi- 

 nation by wave-shock, the upward movement is determined by the 

 presence of the algal cells and their light requirements. The egg- 



Willey, A., Convergence in Evolution, London, Murray, 1911. 



