36 



Marine Invertebrates 



illuminated from above. To avoid over-heating, light from a iooo watt 

 gas-filled bulb was passed through a layer of water. A screen (O) made 

 of oiled paper was inserted to provide uniform illumination. 



METHODS OF OBTAINING A CULTURE OF A 

 SINGLE SPECIES 



Two methods have been employed by Miquel in obtaining cultures of 

 a single species of diatom. The method of isolation consists in 



picking out an individual cell under the micro- 

 scope and introducing it into a prepared 

 medium. If the method of subdivision is 

 used, a small quantity of water containing a 

 mixture of various organisms is added to a 

 prepared medium and poured out into a num- 

 ber of tubes. If the operation is repeated 

 many times some of the tubes may contain 

 one unit of diatoms only from which a fresh 

 culture may be made. 



Allen and Nelson (ioio) proposed the fol- 

 lowing modifications of this procedure. One 

 or two drops of plankton are added to 250 cc. 

 of a suitable sterile medium and poured into 

 petri dishes. The dishes should be kept under 

 a constant temperature in a subdued light, in 

 a place where they may be examined with a 

 hand lens without moving or disturbing them. 

 In the course of a few days colonies of differ- 

 ent species of diatoms will be seen growing at 

 different spots on the bottom. They may 

 be picked up and transferred to fresh culture media. [See pp. 43, 70.] 

 The isolation should be made as early as possible before all the water 

 is infected by some one organism, either diatom or flagellate. Some- 

 times it is necessary to repeat the process several times before one suc- 

 ceeds in isolating the desired species. To facilitate the process of elimina- 

 tion of flagellates and other micro-organisms Schreiber (1927) recom- 

 mends the use of a device shown in figure 37. The principal part of it 

 consists of a U-shaped glass tube, the middle piece of which is drawn 

 into a capillary ; one arm is made longer than the other. A diatom culture 

 is fed drop by drop into the long arm from which it flows through the 

 capillary into the short arm. Owing to the higher velocity of the current 

 in the capillary, the diatoms are carried into the left arm and accumulate 

 in the lower part of it. The rate of the flow of water through the appara- 



Fig. 37. — The device used 

 for the concentration of 

 diatoms. After Schreiber. 

 D, diatoms. 



