464 ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 



the whole medium a deep red-brown. It flourishes in Miquel sea-water, 

 and its nutrition is evidently autotrophic. In one culture in Mi(juel 

 sea-water inoculated with plankton a number of Coccospheres de- 

 veloped, probably Coccosijhcen.i atlantica, Ostenf. Other flagellates 

 and ciliated infusoria are very commonly met with, such as Bodo, 

 Euplotcs, Euglena, etc., which all seem to depend on the diatoms or 

 other vegetable organisms for their food material. 



IV. THE EEAEING OF MAEINE LAEV^. 



In the rearing of pelagic larval forms of marine animals,* the 

 principle which we have followed has been to introduce into pure, 

 sterile sea-water the larvfc 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 w^e 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. 431), and collecting it in sterilized glass vessels. All instruments 

 and pipettes are sterilized by baking in an oven, and a fresh sterile 

 pipette is used for each operation during the progress 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 fertilize a 

 large number of eggs. Excess of sperm should be avoided owing to 

 its liability to putrefy. After an interval of ten or fifteen minutes the 

 water containing the eggs is filtered through bolting silk of 100 meshes 

 per inch, which just allows single eggs to pass through, whilst keeping 



* See BibUogTai)hy ; especially Grave (26), MacBride (28-30), Doncaster (25), etc. 



