250 THE SEAS 



rotation of life as are exhibited in our waters are absent. 

 In many parts of the world, however, there are dry and 

 rainy periods, monsoons, and other weather conditions 

 that alternate with unfailing regularity during the year 

 and it is more than probable that these secular changes 

 may have their effect upon the life in the sea. 



There are at any rate in tropical, as well as temperate, 

 waters animals which, if not giving an index of the season 

 of the year, show without fail, by their spawning habits, 

 the times of the lunar months. It is quite a common 

 occurrence to find that marine animals will breed at certain 

 fixed stages of the moon. It has been shown that our 

 common oyster has a tendency to " spat " in much greater 

 numbers during the week following the full and new moon 

 than at any other time. In Egyptian waters also there 

 is a sea urchin that breeds during the nights when the moon 

 is full. But perhaps one of the most surprising cases of 

 this kind is supplied by a small marine worm from the 

 tropical seas near Samoa in the Pacific Ocean, the " Palolo 

 Worm " {Eunice viridis). This worm can be regarded as a 

 veritable sea calendar. All the year round it lives in holes 

 and crevices among rocks and coral growth on the sea 

 bottom (Plate 89). But true to the very day, each year 

 the worms come to the surface of the sea in vast swarms 

 for their wedding dance. This occurs at dawn just for two 

 days in each of the months, October and November, the 

 day before, and the day on which the moon is in its last 

 quarter ; the worms are most numerous on the second 

 day, when the surface of the ocean appears covered with 

 them. Actually it is not the whole worm that joins in 

 the spawning swarm. The hinder portion of the worm 

 becomes specially modified to carry the sexual products. 

 On the morning of the great day each worm creeps back- 

 wards out of its burrow, and when the modified half is 



