154 



THE ANIfAAL KINGDOM 



ceptor is continually shaded. In general, euglenas swim toward moderate 

 light but avoid intense light. During the day they usually swim to the 

 surface ot a pond where they lorm a green scum, exposing their chloro- 

 plasts to the light. 



No Euglena is completely autotrophic. Healthy cultures can be 

 maintained in light only il some organic substances, especially amino 

 acids, are present. Growth is more rapid if a considerable variety of 

 organic substances is present. In the absence of light, of course, the 

 culture medium must be rich in all the basic foods. Some species, if 

 cultured in the dark, gradually lose their chlorophyll. When the loss 

 is complete, the organisms become obligatory saprophytes, for once 

 chlorophyll is lost it cannot be regained. A few natural species of Eu- 

 glena lack chlorophyll, which suggests that nature may have performed 

 the same experiment in the past. 



Euglenas belong to the order Euglenida, characterized by the gidlet 

 and pigment spot. Some members of the order are nearly autotrophic, 

 some are saprophytic, and some are holozoic. Of the remaining 15 to 25 

 flagellate orders, three will be mentioned here. 



Dinoflagellates. The order Dinoflagellata is characterized by the 

 presence of two flagella in grooves, one trailing posteriorly and the other 

 wrapped around the "waist" (Fig. 8.5). Usually the body is covered by a 

 cellulose shell divided into upper and lower halves. Many species pos- 

 sess photosynthetic pigment and are able to synthesize some of their 

 organic needs. None has been successfully cultured on a completely in- 

 organic medium. Thus, like the euglenas, the dinoflagellates are not 

 completely autotrophic. Some species can be cultured without light if 

 all the necessary foods are supplied. Dinoflagellates have also been ob- 

 served capturing other organisms and engulfing them by pseudopod 

 formation from the ventral furrov*A. It is now believed that most of the 

 species combine autotrophic and holozoic nutrition. Some species lack 

 photosynthetic pigment and live entirely by holozoic nutrition. 



Dinoflagellates are abundant in the plankton of both marine and 



Celloxloge 

 pla.te 



Transverse 

 f IcLgellum. 



■Ventral 

 £urrov/ 



Posterior 

 f la-del lum. 



Figure 8.5. A typical dinoflagellate. The organism propels itself forward with un- 

 dulations of the posterior flagellum, and rotates by activity of the transverse flagellum. 



