246 PROTOZOOLOGY 



in a circle near the anterior end. The longitudinal flagellum often 

 projects beyond the body and vibrates. Combination of the move- 

 ments of these flagella produces whirHng movements characteristic 

 of the organisms. 



The majority of dinofiagellates possess a single somewhat massive 

 nucleus with evenly scattered chromatin, and usually several endo- 

 somes. There are two kinds of vacuoles. One is often surrounded by 

 a ring of smaller vacuoles, while the other is large, contains pink- 

 colored fluid and connected with the exterior by a canal opening into 

 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 pigment-ball. The majority 

 of planktonic forms possess a large number of small chromatophores 

 which are usually dark yellow, brown or sometimes slightly greenish 

 and are located in the periphery 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 bj^ binary or multiple fission or budding 

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

 groups. 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 w^ere found to retain their vital- 

 ity 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 dinofiagellates 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 vari- 

 ous diatoms, copepods and several pelagic animals. 



Bivalve shell without furrows Suborder 1 Prorocentrinea 



Naked or with shell showing furrows. .Suborder 2 Peridiniinea (p. 248) 



Naked; without furrows; no transverse flagellum 



Suborder 3 Cystoflagellata (p. 261) 



Suborder 1 Prorocentrinea Poche 



Test bivalve; without any groove; with yellow chromatophores; 

 2 flagella anterior, one directed anteriorly, the other vibrates in a 

 circle; fresh or salt water. 



