DINOFLAGELLIDA 97 



may run from end to end or from one end to the annulus. The 

 two flagella arise typically from the annulus, one being trans- 

 verse and the other longitudinal. 



The transverse flagellum which is often band-form, en- 

 circles the body and undergoes undulating movements, which 

 in former years were looked upon as ciliary movements (hence 

 the discarded name Cilioflagellata). In the suborder Adinida, 

 this flagellum vibrates freely in a circle near the anterior end. 

 The longitudinal flagellum often projects beyond the body and 

 vibrates. Combination of the movements of these flagella pro- 

 duces whirling movements characteristic of the organisms. 



The majority of Dinoflagellida possess a single somewhat 

 massive nucleus with achromatic network, evenly scattered 

 chromatin, and usually several endosomes. There are two kinds 

 of vacuoles. One is often surrounded by a ring of smaller vacu- 

 oles and the other is large and opens to the exterior by a canal. 

 The latter is known as the pusule, and its function is supposed 

 to be hydrostatic. In many forms a stigma is present, and in 

 some it is provided with an amylaceous lens and a dark pig- 

 ment-ball. The majority of planktonic forms possess a large 

 number of small chromatophores which are usually brownish or 

 often slightly greenish and are located in the periphery of the 

 body. Bottom-dwelling and parasitic forms are often colorless, 

 because of absence of chromatophores. A few contain haemato- 

 chrome. The method of nutrition is holophytic, holozoic, sapro- 

 zoic, or mixotrophic. In holophytic forms, starch and fats are 

 widely distributed. 



Asexual reproduction is by binary or multiple fission in 

 either the active or the resting stage. The mode of division dif- 

 fers among different groups. Encystment is of common occur- 

 rence. In some forms the cyst wall is formed within the test. 

 The cysts remain alive for many years. In one instance, 

 Ceratium cysts were found to retain their vitality after six 

 and one-half years. Conjugation and copulation have been 

 reported in certain forms, but definite knowledge on sexual 

 reproduction awaits further investigation. 



The Dinoflagellida are abundant in the plankton of the sea 

 and play an important part in the economy of marine life as a 

 whole. A number of parasitic forms are also known. Their hosts 



