THE GREEN CELLS OF CONVOLUTA ROSCOFEENSIS. 175 



Batches of Couvoluta were collected in watcli-glasses from 

 the shore, care being- taken to exclude particles of sand and 

 other foreign matter. The animals so obtained were washed 

 a number of times in filtered sea-water, and were then 

 allowed to descend into a large glass di^h containing filtered 

 sea-water. This dish was illuminated unilaterally, and as 

 soon as Convoluta had taken up its markedl}^ positive photo- 

 tropic position the dish was tilted, the water drained away, 

 and the bottom of the dish cleaned. A fresh lot of filtered 

 sea-water was added, and the dish so turned that the animals 

 were impelled to crawl across to the opposite side. As soon 

 as they had taken up their new light-position the tilting 

 and cleaning were repeated, filtered sea-water was again 

 added, and the vessel turned once more. In some experi- 

 ments these processes were repeated as many as eight times. 

 When the Convolutas in the dish began to lay, the egg-capsules 

 were picked out daily and placed in glass vessels containing 

 filtered sea-watei-. As a result of these precautions the 

 capsules were much freer from those organisms which, when 

 no precautions are taken, habitually infest them ; and larger 

 numbers of Convolutas hatched out under these conditions 

 remained colourless. Nevertheless, even with these pre- 

 cautionary measures some green animals ultimately appeared 

 among a great majority of colourless Convolutas. Hence 

 an additional safeguard from possibility of infection had to 

 be employed — namely, to separate the young as nearly as 

 possible at the moment of hatching from the egg-capsules. 

 This, though a tedious, was not a difficult, operation owiug 

 to the fact that when on the point of hatching, three or 

 four days after the capsules have been laid, it suffices, 

 in order to release the hirvse, to take up the capsule in 

 a fine pipette and to eject it with some slight force into 

 the water. Numbers of animals were obtained in this 

 manner, free or almost free from capsule-remnants. Absolute 

 freedom it is almost impossible to obtain, owing to the 

 extreme tenuity and transparency of the capsules at the time 

 of hatching. 



