238 THE AMERICAN MONTHLY [Aug. 



diatom, rendering service in an alternate manner as the 

 diatom advances or retrogrades in its movements. 



Should the formation of the sigmoid lines of black 

 granules not be observed while the diatom is in mo- 

 tion, it will before long come to a halt. Then all at 

 once the sigmoid lines will flash out and indicate their 

 presence by a rapid flow of granules from the direction 

 of the active end, flowing longitudinally towards the 

 passive end. It will be noticed that the sigmoid lines 

 dip into the depressions at the nodular cavities in sharply 

 defined arcs of a circle without coming into contact with 

 each other. After the flow of granules is lost in the 

 nodular cavity, the granules from the first section of the 

 sigmoid lines will be seen to emerge and flow tow^ards 

 the passive end of the frustule where they can be mas- 

 sed or dispersed by the exoplasmic fringe. This melan- 

 orrehic phenomenon may appear as unilateral or bilateral 

 phases, the sigmoid lines having bilateral symmetry. 

 These conditions seem to be governed by the instinctive 

 will of the diatom, as in the case of the vorticellians, 

 starting and arresting their ciliary whorls at their own 

 apparent violition. These color current phases are 

 oftenest seen while the frustule is at a standstill. When 

 movement is again resumed, the currents flow inversely 

 to the previous indications. To prepare for viewing 

 these movements, it is necessary to focus carefully and 

 exactly on the upper margins of the frustular edges 

 when the sutural view or aspect is presented. For, when 

 the valvular or lined face is shown, the current phenom- 

 ena are not of sufficient significance or interest. These 

 have already been detailed. While observing the gran- 

 ular flow on the sigmoid lines, it will be noted that they 

 maintain a constant and fixed relation to the sides of the 

 siliceous frustule for a specific period. A materially evi- 

 dent hyaline space separates the line of granules from 

 the shell. It may be noted that the alate protoplasmic 



