326 University of California Publications in Zoology [Vol.19 



According to these criteria the single flagellura of NoctUuca is the 

 longitudinal one and the transverse one is represented by the degen- 

 erate toothlike or ribbon-like process at the left of the sulcus, while 

 the girdle itself is reduced to a mere trough which branches off to the 

 left side of the sulcus or at a short distance anterior to the posterior 

 flagellum, and dies out quickly on the distended left face (pi. 18, 

 figs. 3, 5, 6). 



This interpretation coincides with the results to be expected from 

 the adaptations to pelagic life manifested by NoctUuca, as shown in 

 (1) great distention by hydrostatic vacuoles, (2) consequent loss of 

 locomotor capacity, and (3) degeneration of locomotor organs. 



The cessation of rotation about the axis and the limitations on 

 progression established by the inflation of the body, reduce the loco- 

 motor organs, and the two flagella, to mere rudiments, and the girdle 

 to but a trace of its proximal end. The sidcus, however, is retained 

 in highly developed form, with its cytostomal function in full opera- 

 tion because of the holozoic nutrition of the organism. In plate 18, 

 figures 1, 2, and 3, are seen anterior, lateral, and ventral views of 

 NoctUuca, so oriented and labelled as to bring out the interpretation 

 here advanced. 



It is not the purpose of this note to discuss the earlier literature 

 on the morphology of NoctUuca or to elaborate this subject. A brief 

 analysis only of the figures will be attempted. In figure 2, plate 18, 

 we have a view of the right side of the body so oriented that the 

 tentacle (tent.) is posterior, the sulcus midventral and longitudinal 

 with a deeply excavated oral pouch (o. p.) containing the cytostome. 

 directed posteroventrally. The suclus consists of two main regions, 

 the more or less deeply recessed oral pouch and cytostome extending 

 from the base of the tentacle anteriorly to the median anterior tip of 

 the recess, and the apical groove (ap. gr.) continued beyond the lip 

 in the median line in the anterior direction. It is a shallow rectilinear 

 groove slightly elevated above the general surface and extending from 

 the anterior lip of the recess on the anteroventral surface to the 

 neighborhood of the apex of the body. 



This tract in its entirety corresponds to the sulcus of Gymnodinium 

 psi udonoctiluca, of Pavillardia tentaculifera and of other dinoflagel- 

 lates. The morphological limits of three regions, precingular or apical, 

 intercingular, and postcingular, of the dinoflagellate sulcus are here 

 recognizable. (1) The apical or precingular section from the proximal 

 junction of the girdle to the anterior end is represented by the region 





