ARTIFICIAL CULTUEE OF MAKJNE PLANKTON ORGANISMS, 417 



IV. The Eearing of Marine Lakvj^. 



In the rearing- of pelagic larval forms of marine animals/ 

 tlie principle wliich we have followed has been to introdnce 

 into pure, sterile sea-water the larvae to be reared, together 

 with a -pure culture of a suitable food. As far as practicable 

 all other organisms have been excluded from the rearing 

 vessels. It should be added that the food used in all successful 

 experiments has been of a vegetable nature, and has continued 

 to grow actively in the vessels. This is important from the 

 point of view of oxygen supply. Under the above conditions, 

 or rather under the nearest approach to them at which we 

 have been able to arrive, no change of water has been found 

 necessary. 



Methods. — It will, perhaps, best make the matter plain 

 if we first of all describe the actual procedure, which we now 

 follow in the case of such an animal as Echinus esculentus 

 or E. acutus. The water to be used is first of all prepared 

 by treating* water from the aquarium tanks with powdered 

 animal charcoal, filtering it through a Berkefeld filter (p. 375), 

 and collecting it in sterilised glass vessels. All instruments 

 and pipettes are sterilised by baking in an oven, and a fresh 

 sterile pipette is used for each operation during the pi-ogress 

 of the work. Specimens of Echinus are then opened until 

 a perfectly ripe female has been found, that is to say, one in 

 which the eggs separate quite freely when a portion of the 

 ovary is shaken in sea-water. 



Pieces of ovary, taken from a little below the exposed 

 surface, are then placed in sterile sea-water in a shallow glass 

 dish, and shaken with forceps in order to get the eggs well 

 separated, or a number of eggs from the centre of the ovary 

 are drawn up with a pipette and placed in the water. A 

 very small quantity of active sperm from a ripe male is then 

 added, very little being sufficient to fertilise a large number 

 of eggs. Excess of sperm should be avoided owing to its 



' See " Bibliography," especially Grave (26), MacBride (28-30), Don- 

 caster (25). etc. 



