THE MASTIGOPHORA 



183 



pigment known as peridinin. Eeproduction takes place by oblique 

 fission (Fig. 11) and by swarm-spores. 1 There are two flagella 

 generally lodged in grooves, of which one traverses the latitude of 

 the body and the other the longitude. The former is called the 

 annulus or girdle and the transverse flagellum plies within it 

 (Fig. 10 (3)). The longitudinal groove is the sulcus harbouring the 

 longitudinal flagellum. 



As already indicated, the Dinoflagellata constitute a very 

 important component of the freshwater and marine plankton, the 

 same generic forms occurring in both media. Moreover, they play 

 an important part in the physiology of oceanic life as a whole. 



FIG. 9. 



Pleoilorina illinoisensis, Kofoid. Colony of thirty-two cells. The four small cells at the 

 .interior pole are the vegetative cells (w), the remainder are facultative parthenogonidia, 

 pp. x 300. (After Kofoid.) 



TRIBE 1. GYJINODIMACEAE. 



There is no cuirass, but the grooves are present. The transverse 

 groove may be semiannular in extent and subcentral in position, with the 

 longitudinal fissure straight and nearly at right angles to it on the ventral 

 side (Hemidinium, Fig. 10 (1)) ; or the transverse groove may form a com- 

 plete ring subterminal in position passing into the ventral longitudinal 

 fissure, the anterior or prae-annular portion being much smaller than the 

 posterior and presenting the appearance of a rostrum (Amphidinium) ; again, 

 the annulus may be complete and occupy approximately the equator of the 

 cell, and the sulcus straight (Gymnodinium) ; finally, both annulus and 

 sulcus may have a spiral twist (Spirodinium, etc.). 



1 Zederbauer (24) has described a process of the fusion of the protoplasm of two 

 individuals of Ceratiitm hirundinella which he regards as conjugation, but as the 

 further history of the zygote (?) has not been traced, it may be only of the nature of 

 plastogamic union such as we find in the Lobosa and Heliozoa. 



