38 Marine Invertebrates 



than 1% of natural seawater was added to the culture medium. Arti- 

 ficial seawater used in Allen's experiment comprised only the six principal 

 salts (see Dittmar's analysis of seawater, page 27) and apparently was 

 deficient in some other growth-promoting substance which is present in 

 natural seawater. 



Other solutions than that of Miquel have been recommended by 

 several investigators for the cultivation of diatoms and green forms. 

 Schreiber (1927) gives the following formula: 



table 9. Medium for cultivation of diatoms and green forms. 



. (Schreiber) 



Potassium nitrate (KNO3) 0.2 gm. 



Dipotassium hydrogen phosphate (K2HPO4) .... 0.1 gm. 



Potassium silicate (K^SiOa) 0.01 gm. 



Ferric sulphate (Fe 2 [S04].3) °-°°5 gm- 



Redistilled water 50.00 cc. 



This solution is added to 950 cc. of filtered seawater and the medium is 

 sterilized by steam at about ioo° C. If during the heating a precipitate 

 (calcium carbonate) is formed, the solution should be allowed to stand 

 several weeks until a sufficient amount of C0 2 has been absorbed from 

 the air and the calcium salts redissolved. 



Although diatoms constitute the most essential food element of a great 

 number of plankton-feeding larvae, other food organisms should not be 

 neglected. Mixed cultures of various green flagellates may be obtained 

 from jars in which the algae are allowed to putrify. These cultures once 

 obtained may be carried on for a long time by inoculating Miquel's or 

 other media. Their usefulness in feeding marine larvae should be 

 ascertained, however, by experimentation. 



Isolation of a green flagellate and its maintenance in a bacteria-free 

 culture presents less difficulty than does the isolation of a diatom. Using 

 standard bacteriological technique, German investigators succeeded, for 

 instance, in obtaining bacteria-free pure cultures of Carteria sp., a phyto- 

 monadine flagellate very common in the North Sea (Schreiber, 1927). 

 The sample of plankton is first centrifuged in sterile seawater and set 

 aside and the Carteriae, which are positively phototropic, are separated 

 from the rest of the planktonic organisms and placed on gelatin or agar 

 plates, from which they are subcultured. 



Sperm of marine algae is often used as food for small lamellibranch 

 larvae which are unable to ingest diatoms. At Woods Hole the sperm 

 of Ulva may be obtained during the summer. Freshly collected plants 

 are left exposed overnight on pieces of filter paper. The next morning the 

 leaves are put in a shallow crystallizing dish filled with water and set 

 in a place exposed to strong light. In a short time large masses of green 

 sperm may be pipetted from the side having the greatest illumination. 



