peridinin, which is responsible for the brown color exhibited by the 

 autotrophic forms. Marine holozoic organisms are commonly red, 

 purple, or yellow. In some groups there are pyrenoids. Reserve food 

 ordinarily collects as starch, although in their nutrition these or- 

 ganisms may be holozoic, holophytic, or saprophytic. 



In most of the free-swimming species the protoplast contains a 

 red pigment-spot which is unusually large and conspicuous and may 

 be either simple or complex with lens-like structures. 



The cell is either a naked protoplast or is inclosed by a thin or 

 thick wall. In the latter case (the armored dinoflagellates ) , the 

 envelope is complex, being composed of a varying number of plates, 

 which may be smooth or rough and spiny. The size, arrangement, 

 and number of these plates are of taxonomic value. 



The motile forms have 2 flagella attached on the ventral surface 

 in a more or less conspicuous longitudinal furrow or sulcus. One of 

 these trails behind the cell while the other is wrapped about it in 

 a transverse furrow in which the flagellum vibrates, causing the 

 organism to rotate on its axis as it swims forward. The transverse 

 furrow marks the juncture of the two halves of which the wall (in 

 most forms ) is composed. In the anterior part of the cell ( epicene ) , 

 the plates comprise the epitheca, and those in the posterior part 

 (hypocone) form the hypotheca. See Eddy (1930) for figures il- 

 lustrating plans of arrangement and nomenclature of the plates. 



In the epitheca the plates adjoining the transverse girdle are 

 called the precingulars. Not always present are a few anterior 

 intercalary plates interspersed between the precingulars and the 

 plates at the apex which are known as the apicals. Correspondingly, 

 the plates of the hypotheca adjacent to the girdle are called the 

 postcingulars, with 1 or 2 antapicals at the posterior pole. Rarely 

 there may be a posterior intercalary plate between the postcingulars 

 and the antapicals. In addition to the components mentioned, there 

 may be a more or less prominent ventral plate which lies just above 

 the anterior end of the longitudinal furrow. It may or may not extend 

 to the anterior pole. 



When viewed from the ventral side so that the longitudinal 

 furrow is seen, the right side of the organism is on the left of the 

 observer, and is at the right of the observer, of course, when seen 

 dorsally with only the transverse furrow in view. 



In one order, Dinococcales, the cells are not motile in the vegeta- 

 tive phase and do not carry on cell division. Unlike members of the 

 other orders they reproduce by forming 2-8 autospores or zoospores. 

 It is significant that the sedentary cells are sometimes very similar 



[424 ] 



