v] NATURE OF PL ANT- ANIMALS 151 



were identical except for the fact that one lot received 

 uric acid. The animals supplied with no extra nitrogen 

 laid nine clutches of eggs, whereas the animals sup- 

 plied with extra nitrogen laid twenty-seven clutches. 

 The results of the two sets of experiments just 

 described serve to account for the rich development 

 of algal cells within the bodies of the plant-animals. 

 In their free state, these algee, like all marine plants, 

 run grave and frequent risk of nitrogen- starvation, 

 or at all events of having their increase limited by 

 the shortage of available nitrogen in the sea. Wherever 

 there is any leakage of nitrogen compounds and 

 traces of combined nitrogen must be given off from 

 such animals of C. roscoffensis and C. paradoxa 

 marine, motile plants will congregate. Congregating 

 about our plant-animals, such minute organisms are 

 ingested indifferently. Out of this mixed infection C. 

 roscoffensis and C. paradoxa make each a pure culture, 

 the one of green cells the other of yellow-brown cells. 

 Established in the body, the algal cells find them- 

 selves transferred from a region of scarcity to a land 

 of plenty. Outside, in the open sea, the amount of 

 nitrogen available is but small and the claimants for 

 a share of it innumerable : within the body, the 

 amount of suitable, combined nitrogen is large and 

 at the exclusive disposal of the algal visitors. In 

 such Capuan circumstances, the algal cells grow and 

 divide luxuriantly. Their photosynthetic activities 



