50 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA 



others are ectoparasitic, as Oodinium poucheti on Appendiciilaria (Pouchet, 

 1885o). 



The dinoflagellates are thus found to have evolved all the types of nutrition 

 that are known in other groups of living organisms. The manner in which 

 this has taken place is difficult to conjecture, since the fact that all types may 

 be found in the same plankton haul, under precisely similar conditions, pre- 

 cludes an explanation of environmental changes or isolation as the determining 

 factor. Neither are there any evidences which would make sexual selection 

 responsible for the change. The factors which afford a plausible explanation 

 of these phenomena in the Metazoa are difficult to apply to these simply 

 organized creatures without apparent sexual differentiation or behavior. 



The possible relation of the process of digestion and the color of the organ- 

 ism is shown in the characteristic color that is found in what appear to be food 

 balls in some species. In many cases these bodies vary in size and often in 

 position in individuals of the same species with relatively little variation in 

 color. These are evidently not secretions, such as starch grains, etc., but appear 

 to be food bodies in process of solution. The characteristic color for the species 

 may possibly be due to the specific chemical reactions of the digestive fluids, 

 and as such maj^ vary but slightly for different members of the same species. 



The presence of a definite mouth or cytostome in the Dinoflagellata is ob- 

 scured by the dislocation of the flagella from the functional anterior end of 

 the body and their migration to a midventral location. It is still more modified 

 and seemingly obliterated in many species by the separation of the two flagella 

 and the resulting presence of two distinct flagellar pores, the anterior one from 

 which the transverse flagellmn takes its origin, and the posterior from which 

 the trailing, propelling flagelhim emerges. Between these two lies the inter- 

 cingular part of the sulcus and from each of the pores a slender canal leads to 

 an expanded pusule. These pusules may have their deeper ends approximated 

 or even joined in a continuous cavity from pore to pore. 



While no dinoflagellate has been seen by us to capture food with the lips 

 of the iutercingular siilcus it appears from its relation to the pores and flagella 

 and its probable derivation from the cytostomal region of more primitive flagel- 

 lates that this region is the morphological equivalent of the mouth or cystostome 

 and functions as such in the capture of the relatively large organisms found 

 within its food A^acuoles. The astonishing plasticity of the oral region observed 

 by us in some parasitic trichomona ds, and reported by Ehodes (1920) in 

 C ollodictijon , encourages us to hazard the inference that the interciugidar sulcus 

 is correspondingly mobile and efficient as a food-capturing mechanism in tlie 

 unarmored dinoflagellates. From the evolutionary standpoint it has been one 

 of the most ])lastic parts of the organism and it is perhaps equally mobile in 

 the individual. 



Eeactioxs to Stimuli. — This is still an imtouched field as far as the dino- 

 flagellates are concerned, as it is for the majority of Protozoa in general. 



