320 University of California Publications in Zoology [ VoL - 19 



The former encircles the body transversely or in a descending left 

 spiral of 0.5 turn in Hemidinium and Oxyrrhis (fig. B), for a full 

 turn in Gymnodinium (figs. C, D), for not to exceed 1.5 turns in 

 Gyrodinium (fig. A), and up to 3 or 4 turns in Gochlodinium (fig. E). 

 Except in Hemidinium and Oxyrrhis its distal end joins the sulcus. 



The girdle in dinoflagellates is occupied by the transverse flagellum 

 (fig. A, tr. ft. ). a broad ribbonlike flagellum, which passes from its 

 origin at the junction of girdle and sulcus on the ventral face, distally 

 in the girdle for the whole or a part of its length. It is free except for 

 its attachment at its base, but in life nomially lies in the girdle, where 

 short waves of contraction traverse it distally. When fixed by pre- 

 servatives it may be thrown out of the girdle in close-set undulations 

 (fig. I). 



The sulcus (fig. A, side), on the other hand, is longitudinal and 

 extends for a. varying distance in the morphological midventral line 

 between the apex and antapex of the body. As the body undergoes 

 torsion in the higher genera, such as Pouchetia and Gochlodinium, 

 the sulcus is involved in the torsion and makes one less turn than the 

 girdle. The sulcus always contains the two flagellar pores. The 

 anterior one. from which the transverse flagellum arises, is at or near 

 its junction with the proximal end of the girdle, and the posterior 

 pore, from which the posterior or longitudinal flagellum arises, is at 

 its junction with the distal end of the girdle or below it. 



The anterior end of the sulcus may extend far anterior to the 

 girdle to the very apex, or beyond it, and, in species with torsion, this 

 anterior extension of the sulcus may be continued in a spiral, apical 

 loop about the apex. In like manner the posterior section of the 

 sulcus below the distal end of the girdle may be carried around the 

 antapex in an antapical loop continuing the torsion, though it usually 

 runs directly posteriorly in the midventral line. The longitudinal or 

 posterior flagellum extends distally in the sulcus and projects beyond 

 the posterior end of the body. In contradistinction to the transverse 

 flagellum it is a slender thread not thrown in close-set undulations. 

 The intercingular part of the sulcus, between the two ends of the 

 girdle, is increased in length by the posterior displacement of the 

 distal end of the girdle, especially in the higher genera, such as 

 Gyrodinium (fig. A) and Cochlodinium (fig. E), where there is much 

 torsion. It is mainly through the sulcus, especially its intercingular 

 section, that holozoic species of Gymnodinioidae engulf the organisms 

 upon which they feed. It. is thus the homologue of the cytostome of 



