328 University of California Publications in Zoology [Vol. 19 



The degeneration of all the girdle except its proximal end appar- 

 ently accompanies the complete disappearance of the transverse ribbon- 

 like flagellum in the zoospore stage in which the longitudinal flagellum 

 is present and functional. There appears, however, to be an organ 

 in the adult which, from its position, variability, and structure, may 

 represent the degenerated transverse flagellum. This organ is the 

 so-called "tooth" of Huxley (1855), or "prehensile organ" of Bright- 

 well (1857). It is a projection of the surface, at or in the proximal 

 end of the girdle on the left side of the sulcus. In life it forms an 

 overarching projection anterior to the base of the longitudinal flagel- 

 lum. It varies greatly in outline, having sometimes a row (pi. 18, 

 fig. 3) of three or more ecpiidistant ecpial prominences significantly 

 suggestive of undulations of the transverse flagellum. 



In other cases there may be a single prominent projection and 

 other minor ones passing distally into the girdle. In a few instances 

 observed by me and rarely shown in published figures, this structure 

 appears to stand out as a long projection whose length is from several to 

 as many a.s ten times its width. This appears in two of Huxley's (1855, 

 pi. 5, figs. 2. 5) figures. It may be the structure which Adman (1872, 

 pi. 18, fig. 2, reproduced in modified form in our pi. 18, fig. 2) figures 

 as a tube, its vibratile( ?) action suggesting to him his interpretation. 

 Traces of it appear in Brightwell's (1857, pi. 12, figs. 9, 13) figures 

 and in those of Cienkowsky (1873, pi. 3. fig. 1, 6). 



The interpretation of this toothlike organ as the transverse flagellum 

 is significantly confirmed in Ishikawa's (1899) account of mitosis. 

 This investigator shows that this structure persists as a prominent 

 projection during mitosis and, in common with the longitudinal 

 flagellum, arises immediately adjacent to the large centrosphere of 

 "archoplasm" (see his pi. 19, figs. 1, 4). exactly where a flagellum 

 would be expected to have its origin. The morphological evidence is 

 plainly conclusive that the tooth is the degenerate transverse flagellum. 



The evidence from the functional standpoint is, perhaps naturally, 

 less convincing. Movements in this organ, as I have thus far observed 

 them, are slow and spasmodic. No rapid undulations have been seen 

 by me but only very slow almost imperceptible retractions of the 

 projections, and more rapid, spasmodic, somewhat rhythmical, down- 

 ward beats into the sulcus on the part of this overarching projection 

 or flagellum. The rhythmical feature is suggestive of flagellar activity. 



The morphological evidence is thus conclusive that Noctiluca is a 

 dinoflagellate highly modified through distention by hydrostatic 



