224 



THE EVOLUTION OF THE METAZOA 



does not go so far, as has been shown by recent researches, 

 their animal food is usually consumed during the night and 

 therefore during the day-time we do not find any food of ani- 

 mal origin in the gastrovascular system of polyps belonging 

 to corals. It seems that the sessile way of life favoured the 

 stabilization and further development of this mutual relation- 



Fig. 41. A section of the egg-cell of Halecium sp. with symbiotic 

 zooxanthellae pervading in the egg (from Hadzi). 



ship, so that finally these algae could be regularly passed over 

 to the eggs, a development which was facilitated by the posi- 

 tion of ovaries and by their approachability. It was more than 

 40 years ago that I was able to show how zooxanthellae actively 

 penetrate into the young egg cells of a species belonging 

 to the hydroid genus Halecium (Hadzi, 1912 Fig. 41). At 

 about the same time (1906) I succeeded at the then established 

 "Biologische Versuchsanstalt in Wien," which w^as headed by 

 the well-known experimental zoologist Hans Przibram, in 

 making green Hydra colourless, after I had reared the sexu- 

 ally ripe animals in complete darkness (the same experiment 



