188 F. KEKBLE AND F. W. GAMBLE. 



tive testimony^ since, in cultures containing disintegrating 

 Convolutas, thoy take up positions among tlie bretiking-down 

 green cells of the animal and increase greatly in numbers. 

 Again, the rapid increase of the alga which follows after 

 infection is strong evidence of saprophytic powers of nutrition. 

 Finally, we have proved by comparative cultures that the 

 infecting alga increases by vegetative division quicker when 

 supplied with organic nitrogen than when the nitrogen is in 

 the form of nitrates. The details of these experiments are 

 given in Table IV, which summarises the results of a series 

 of cultures of the alga in sterile sea-water, to which nitrogen 

 iu different combinations was added. The most rii})id increase 

 took place in the culture containing uric acid, next in 

 that containing urea, the increase in sea-water containing 

 jjotassium nitrate being less than in either of these culture- 

 fluids. Tlie infecting organism, then, has even in its green- 

 celled stage marked saprophytic tendencies — tendencies which 

 appear to hud full expression in the colourless stnge and 

 which offer the key to the origin of the remarkable associa- 

 tion of green cell and Convoluta. 



Two questions remain — (1) Is there any special adaptation 

 in the habits of the infecting organism which brings it into 

 close association with Convoluta, or is its prevalence in the 

 surface slime of the animal and in the empty egg-capsules a 

 matter of chance ? (2) To what systematic position is this 

 organism to be assigned ? 



To the first question the following experiments provide 

 a decisive answer. If to a bulk of filtered sea-water con- 

 taining egg-capsules which have been laid under the cleanest 

 possible conditions a number of the flagellated cells are 

 transferred by means of a platinum loop, then within a few 

 hours one or more of these cells Avill be found to have 

 settled down on each capsule. Yet more striking results 

 are obtained if to a hanging-drop containing an vgi^- 

 cai)sule with its gi-oup of developing eggs a number of 

 the flagellated cells are added. In sueh a ])reparation 

 the motile green cells are seen to aj)proach the capsule, to 



