90 MEMOIKS OF THE UNIVERSITY OP CALIFORNIA 



wall a furrow slightly wider than the amplitude of its vibrations. A similar 

 origin for the longitudinal furrow or sulcus seems evident. In the simpler 

 forms both llagella arise from or near the same point (figs. X, 2, 14), hence we 

 find the sulcus extending from the girdle to the posterior end of the body, with 

 a gradual widening posteriorly to accommodate the movements of the flagellmn. 

 With the deepening of this furrow a secondary modification is introduced, in 

 that the depression extends anteriorly above the girdle. In Gymnodinium 

 hifurcatum (fig. AA, 3), where the sulcus bifurcates the h\'pocone, the flagellum 

 is found to swing freely through the cleft thus made, often projecting from the 

 body from the dorsal side, instead of from the usual ventral position. 



With the continued increase in the length of the transverse flagellum the 

 length of the girdle has kept pace with it. This, combined with the rotation of 

 the body, has resulted in a spiral course for the girdle with its distal end dis- 

 placed posterioi'ly, and gradually pushed farther and farther around the body. 

 The culmination of this lengthening is seen in CocModinium, where, in its most 

 specialized species, the girdle may make no less than four turns ai'ound the body. 



The continued lengthening of the sulcus in these highly specialized forms 

 was not necessary to accommodate the longitudinal flagellum, which has retained 

 a position near or slightly posterior to the distal end of the girdle. Tlie conse- 

 quence is that this flagellum is not involved in the resulting torsion of the body 

 in the intercingular region. In the meantime, with the development of holozoic 

 nutrition, the intercingular part of the sulcus had become the area for the 

 ingestion of food, and hence was necessary in the economy of the organism. 

 Its development, therefore, kept pace with that of related structures. In the 

 Peridinioidae elaborate fins, or lists, further outline both girdle and sulcus, and 

 guide the currents of water which course through them. 



In the arenaciphilous group of dinofiagellates certain features are possessed 

 in common by most of the species, which probably owe their development to 

 the influence of their habitat. These forms range in length from 17/^ to 56m, a 

 size which is considerably below the general average for the G}^nnodinioidae 

 as a Avhole. This small size is evidently an adaptation to the habitat in which 

 the organisms live, enabling them to move more readily through the interstices 

 between the grains of sand and to escape, to some extent, its crushing effects 

 during its not inconsiderable movements. Another adaptation for the same 

 pur]iose is the strong dorsoventral flattening of the body, giving to it a thin 

 disk shape that is very rarely met with in the characteristically pelagic or 

 neritic forms. Species showing these modifications are foimd in three arena- 

 ciphilous genei-a, as in Aniphidinmm steini and A. scissum (figs. C, 2, 3), 

 Gymnodinium agile (fig. Y, 9), and Gyrodininyn viridescens (fig. DD, 11). The 

 deep sulcal groove with its overhanging fiaps is utilized as a holdfast, an 

 adaptation to the conditions of life in shifting sand. 



Another significant fact is that most of the species found in this habitat 

 are green in color. This is perhaps correlated with the fact that the beach at 



