DINOFLAGELLATA 217 



years were looked upon as ciliary movements (hence the name 

 CiHoflagellata). In the suborder Adinida, this fiagellum vibrates 

 freely in a circle near the anterior end. The longitudinal fiagellum 

 often projects beyond the body and vibrates. Combination of the 

 movements of these flagella produces whirling movements char- 

 acteristic of the organisms. 



The majority of dinoflagellates possess a single somewhat mas- 

 sive nucleus with evenly scattered chromatin, and usually several 

 endosomes. There are two kinds of vacuoles. One is often sur- 

 rounded by a ring of smaller vacuoles, while the other is large 

 contains pink-colored fluid and connected with the exterior by a 

 canal opening in a flagellar pore. The latter is known as the pusule 

 which functions as a digestive organella (Kofoid and Swezy). In 

 many freshwater forms a stigma is present, and in Pouchetiidae 

 there is an ocellus composed of an amyloid lens and a dark pig- 

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

 ber of small chromatophores which are usually dark yellow, 

 brown or sometimes slightly greenish and are located in the pe- 

 riphery of the body, while bottom-dwelling and parasitic forms 

 are, as a rule, colorless, because of the absence of chromatophores. 

 A few forms contain haematochrome. The method of nutrition is 

 holophytic, holozoic, saprozoic, or mixotrophic. In holophytic 

 forms, anabolic products are starch, oil, or fats. 



Asexual reproduction is by binary or multiple fission or bud- 

 ding in either the active or the resting stage and differs among 

 different goups. Encystment is of common occurrence. In some 

 forms the cyst wall is formed within the test. The cysts remain 

 alive for many years ; for example, Ceratium cysts were found to 

 retain their vitahty in one instance for six and one-half years. 

 Conjugation and sexual fusion have been reported in certain 

 forms, but definite knowledge on sexual reproduction awaits 

 further investigation. 



The dinoflagellates 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 include 

 various diatoms, copepods and several pelagic animals. 



Bivalve shell without furrows Suborder 1 Prorocentrinea (p. 218) 



Nakes or with shell showing furrows 



Suborder 2 Peridiniinea (p. 219) 



Naked; without furrows; no transverse fiagellum 



Suborder 3 Cystoflagellata (p. 233) 



