1920] Kofoid: New Tnteryyn tation of the Structure of Noctiluca 327 



included in that part of the cytostome anterior to the girdle, phis the 

 apical grove. (2) The intercingular part of the sulcus extends pos- 

 teriorly from the girdle. Its posterior limit is undefined, owing to the 

 failure of the girdle to make the circuit of the body. Because of the 

 fact that the posterior flagellum, as a rule, originates at or near the 

 junction of the distal end of the girdle with the sulcus, it may be 

 assumed that the base of this flagellum in Noctiluca represents approxi- 

 mately the posterior limit of the intercingular sulcus, which normally 

 is found between the junctions of the proximal and distal ends of the 

 girdle with the sulcus. In Noctiluca this region is very short, indicat- 

 ing that there is little, if any, "of the displacement which normally 

 forms the intercingular region. (3) The remainder of the sulcus 

 between the base of this flagellum and the tentacle is the homologue 

 of the antapical or postcingular section of the sulcus. 



The girdle is best seen in the ventral view (pi. 18, figs. 3, 5, g.) . It 

 is a short trough, making less than 0.25 of a turn around the body in 

 the smaller individuals and often obliterated in the larger ones. It 

 originates very near, or slightly anterior to, the base of the longi- 

 tudinal flagellum (see Robin, 1878, pi. 37, fig. 6; pi. 41, fig. 24; repro- 

 duced in our pi. 18, fig. 5). It is narrower than the sulcus and leaves 

 it in an arching curve, convex anteriorly, which narrows and fades 

 out distally before reaching the left side of the body, that is, in about 

 0.08 to 0.20 turn in Robin's (1878, pi. 36, fig. 4; pi. 37, fig. 6; pi. 41, 

 fig. 24) smaller individuals, 400-425 microns in diameter. 



This structure has precisely the morphological relations of the 

 proximal end of the girdle and its characteristic form, but has never 

 before been noted or interpreted, though sketched without comment by 

 two investigators. It was clearly figured by Robin (1878) but he calls 

 no attention to it and does not designate it. A bit of its proximal end 

 appears in one only of Webb's (1855, pi. 6, fig. 6) figures but without 

 designation or comment. There appears to be no other suggestion of 

 this structure in the literature of the subject. 



The two fiagella typical of the dinoflagellates are both represented 

 in Noctiluca. The longitudinal flagellum is the short, threadlike, 

 vibratile structure in the sulcus trailing posteriorly from its origin in 

 the depths of the sulcal recess a short distance posterior to the junction 

 of the proximal end of the girdle with the sulcus. It is relatively small 

 with respect to the size of the body, and appears to have little if any 

 appreciable effect on the movements of the body. 



