52 



Peridiniesp- 



region. Vacuoles of considerable size may be present in the cytoplasm, 

 through which branched protoplasmic threads pass to the peripheral part 

 of the protoplast. The nucleus, which is of comparatively large size and 

 somewhat variable in shape, may be anterior in position (as in Gymnodinium 

 fuscum) or posterior (as in Hemidinium}. The chromatin granules are 

 scattered, and on division there is a primitive mitosis. 



A red pigment-spot is present in some forms, but wanting in others. 

 In the genus Pouchetia there is a lens-like body associated with the pigment- 

 spot, and it is assumed that the concentration of light upon the pigment-spot 

 by this body largely influences the orientation of the cell. Trichocyst-like 



^.. : '* : \ 1 



/.:*&* *--v-- chr. 



tr.f 



Fig. 37. Lateral view of Amphidinium sulcatum Kofoiil. ami/., amyloid body; chr., chromato- 

 pliore; ep., anterior part of cell in front of transverse furrow; fl.p., flagellar pore; /./., 

 longitudinal furrow; ., nucleus; tr.f., transverse furrow (with flagellum); vac., vacuole. 

 x 740 (after Kofoid). 



structures occur in certain species of Gymnodinium and Spirodinium, some- 

 times forming a more or less continuous peripheral zone. Some species 

 of these genera throw out numerous threads of mucus, either just previous to 

 entering the resting state or on the advent of unfavourable external con- 

 ditions : these threads rapidly swell up and form a colourless gelatinous 

 envelope. 



The chromatophores are disc-shaped, rod-like, or band-like, and are 

 mostly of a golden-brown colour, although Gymnodinium viride is green, 

 G. teruginosum is blue-green, and G. cceruleum blue. The two latter organisms 

 contain phycocyanin in addition to the green and brown pigments. The 



