16 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA 



torsion of the body is the torsion of the sulcus which of necessity is carried 

 around the body in a spiral course, usually one turn less in length than that of 

 the girdle (figs' GG, HH). 



The increased length of girdle and sulcus, with the consequent torsion of 

 the body, results in a profound modification of its dorsoventral plane. In 

 Gpmnodiniion, which lacks appreciable torsion, the plane passing through the 

 two flagellar pores marks the dorsoventral plane of the body and is longitu- 

 dinal. These two pores are usually located at the junctions of the girdle and 

 sulcus, the anterior pore at the anterior junction, the posterior pore at the 

 posterior junction. The latter pore may frequently open into the sulcus 

 posterior to the junction, but only rarely anterior to it. 



In the simj)ler G}^nnodinioidae these two pores are placed near together, 

 as in OxyrrMs (fig. R, 3), Hemidinium (fig. R, 4), many Ampliidinium (figs. 

 U, 2, 3, 10, 25), and Gymnodinium (figs. X, 2, 14). They still lie, however, 

 in a longitiidinal plane passing through lioth apices. In some species of the 

 last two genera the pores have become widely separated (figs. U, 1, 4 ; X, 5 ; AA, 

 6), a condition which is common for the more specialized genera, as Gyrodinium 

 (figs. CC) and CocModinium (fig. GG), as well as in the Pouchetiidae, where 

 the ends of the girdle are more or less widely displaced. In these species, where 

 ai^preciable torsion of the body is found, the morphologically dorsoventral plane, 

 passing through the two pores, becomes correspondingly twisted, and ceases to 

 lie in the geometrical longitudinal plane passing through both apices. 



With the gradual increase in the length of the girdle in Gyrodinium and 

 Coehlodinium the posterior end of the girdle is pushed farther around the 

 body, carrying with it the posterior pore and sulcus, as well as the morphological 

 ventral surface lying between the two pores. With the continued increase of 

 the girdle up to two complete turns of the body, the morphological dorsoventral 

 plane imdergoes a corresponding torsion with these structures, although the 

 general biconical or fusiform shape of the body as a whole differs little if at 

 all from that of the non-twisted forms. As a result of the torsion, the ventral 

 surface established by the presence of the sulcus follows the torsion of this 

 structure in its one to four turns (as in Coclilodinium augustum) around the 

 longitudinal axis of the body. Thus the position of the anterior pore alone does 

 not determine the ventral face of the organism. 



The sulcus represents the most mobile, plastic portion of the organism. It 

 is the region for the ingestion of food and hence is capable of great distension, 

 judging by the size of the organisms sometunes ingested. In Coehlodinium 

 roseaceum (pi. 8, fig. 85) the ingested Ponclietia has a length of 0.48 of the 

 length and a breadth of 0.33 of the trausdiameter of the Coehlodinium which 

 has mastered it. In C. vinctum (pi. 2, fig. 15) the food mass contained within 

 its body has a length of 0.7 and a width of 0.41 of its own dimensions respec- 

 tively. A still more striking instance is found in Pouchetia voraeis (fig. PP, 2), 

 where a thecate Peridinium has been successfully captured. The cytoplasm 



