PHYTOMASTIGINA 49 



together for a considerable time as a chain. The occurrence of syn- 

 gamy is suspected but has not yet been proved beyond doubt. 



The typical members of this order are free-living and highly 

 organized, but it includes forms which are greatly degenerate and 

 only recognizable as belonging to it while they are spores. The 

 members may be holophytic, saprophytic, or holozoic, feeding by 

 pseudopodia either from a spot on the sulcus or at any point. They 

 are usually pelagic, sometimes parasitic, and for the most part marine. 



Ceratium (Fig. 40 A). Typical, armoured, holophytic species; 

 with three long spines. In freshwater forms the chromatophores are 

 green; in marine species they are yellow or brown. 



Dinophysinae. Pelagic genera, often of bizarre form, with the 

 annulus at one end of the body, and the shell in two lateral plates. 



Polykrikos (Fig. 40 B). Soft-bodied species; colourless and holo- 

 zoic; with the flagella and other external features repeated several 

 times along the axis of the body, and the nucleus also repeated, 

 but not in correspondence with the other features (see p. 6). The 

 protoplasm contains peculiar nematocyst-like organs. Holozoic. 



Oodinium (Fig. 34). Thin-cuticled ; pear-shaped ; colourless ; living 

 as an ectoparasite on marine pelagic animals, and possessing the 

 typical dinoflagellate organization only in the spore stage. 



Dinamoebidium. Colourless and holozoic; completely Amoeba-\ik.Q 

 in the ordinary phase, but forming dinoflagellate swarm spores in a 

 fusiform cyst. 



Noctiluca (Fig. 41). (Formerly placed in an independent order — 

 Cystoflagellata.) Large, peach-shaped forms; colourless and holo- 

 zoic; with highly vacuolated protoplasm; a stout pellicle; and, in 

 the groove of the peach, an elongate mouth, a small flagellum, a 

 structure known as the tooth which is said to represent the transverse 

 flagellum, and a strong tentacle, homologous with a similar structure 

 in certain more normal dinoflagellates. The animal is phosphorescent. 

 Like other dinoflagellates it reproduces by binary fission and by spore 

 formation after multiple fission. The spores are more dinoflagellate- 

 like than the adult. Marine, pelagic. 



Dinothrix, Normally in the palmella phase, forming thread-Hke 

 growths. Marine. 



Order VOLVOCINA 



Phytomastigina which have usually a flask-shaped, green chromato- 

 phore, with one or more pyrenoids, but are sometimes colourless, 

 though never holozoic; form starch reserves, even when colourless; 

 have no gullet or transverse groove; possess usually a cellulose 

 cuticle and often haematochrome ; and regularly undergo syngamy. 



