OF THE HYDROMEDUS^. 399 



many of the eggs fail to develop if no males are present. Eutiina is very regular in its 

 breeding habits, and while my specimens were captured at all hours in the day, nearly 

 all the eggs were laid between the hours of 7.30 and 8.30 p. m. 



The tendency to lay eggs at a fixed hour of the day seems to be quite prevalent 

 among marine animals, and a knowledge of this is of the greatest importance to natu- 

 ralists, since a failure to procure the fertilized eggs of an animal may often be due to 

 the fact that it has not been collected ov observed at the proper hour. The phe- 

 nomenon has received very little attention and I therefore give a few illustrations which 

 have recently attracted my attention. Clans in 1882 (14) and Merejkowsky in 1883 

 (50) have shown that the young stages of yEquora and Obelia are found only in the 

 morning, and Merejkowsky says that the successive steps in the formation of the planula 

 of Obelia follow each other with such regularity that each stage is met with only at a 

 definite hour in the morning. This author attributes the regularity to the direct influ- 

 ence of light, but he gives no proof of this and observations which have been made at 

 Beaufort, under my direction, during the past three or four years, show that the perio- 

 dicity is not due to any external influence, but that it is a specific characteristic deter- 

 mined within the organism. Wilson found at Beaufort that the eggs of Benilla, an 

 Alcyonarian, which lives in the sand below low-tide mark, are always laid at or about 

 6 A. m. He observed only a single instance of spawning at 5.30 and it was never ob- 

 served later than 7 A. M. The regularity is entirely independent of temperature, for 

 the spawning hour was the same on cold and on warm days, although the rate at which 

 the embryo develops does vary with the temperature. He says that the eggs of 

 Leptogorgia are laid with the same regularity, although in this case the hour is 4 A. m. 

 (67). 



While Merejkowsky says that the eggs of Obelia are laid early in the morning, I find 

 that several allied Beaufort medusae spawn at night. Thus Eutima, Eirene, Turritop- 

 sis and Liriope discharge most of their eggs about 8 p. m., although captive specimens 

 drop a few eggs irregularly at all hours. As one hydromedusa lays its eggs early in 

 the morning, while other species lay them in the evening, the regulating influence can 

 hardly be the supply of light. 



While studying the development of a pelagic crustacean, Lucifer, I found that sexual 

 union took place with great regularity, between 6 and 8 p. m.; while the eggs were 

 laid between 8 and 10 p.m., so that the early stages can be studied only between 10 p. m. 

 and 7 A. m. 



Dr. H. H. Donaldson has observed at Beaufort that actinias of various genera are 

 fully expanded only between the hours of 5 and 6 p. M. This is true of these animals 

 in their natural homes, as well as in aquaria; and experiments showed that specimens 

 which were kept in darkness expanded as promptly at the proper hour as those which 

 were exposed to direct sunlight. 



Among the animals which are here enumerated, some live at the surface, as Eutima 

 and Obelia; some, such as the actinias, live near low-tide mark; some, Renilla for ex- 

 ample, live in deeper water; and some, like Lucifer, are vigorous swimmers, while some, 

 like Geryonia, are fixed. Wilson's observations show that the periodicity is not due 



