NAVICULA SOCIALIS. II7 



that over nine groups in ten show this alternation, and thinks 

 all the circumstances point to a common origin and a common 

 experience for each frustule of a given group. 



Of prime importance, and to my thinking, of surpassing 

 interest, is the smooth cooperation of the four cells in locomo- 

 tion. The group proceeds in an orderly manner, and as by 

 common consent, without more appearance of cross-purpose 

 between its members than is to be seen sometimes between 

 the two ends of the same cell in isolated diatoms. For 

 the most part, the tabular groups move along the smooth 

 surface of the slide in straight or gently curving lines, 

 making trails quite similar to those already mapped in 

 these Proceedings'^ for isolated frustules. On occasion the 

 motion is reversed, or again the group may revolve on 

 one corner as a pivot (owing to a sudden spurt of activ- 

 ity in one or two cells perhaps) and so start off in a new 

 direction. There seems to be quite as much cooperation, quite 

 as much simultaneous effort in the same sense, as is observed, 

 for instance, in the frustules of a group of Nitzschia paradoxa. 

 But unlike that diatom, A^. socialis continues for long periods 

 moving in the same direction, resembling in this its own rela- 

 tives in the genus Navicula, but still more Eunotia major. 



The common purpose, if one may so name it, sometimes 

 shows itself in unexpected and surprising ways. When a 

 group meets a firm obstacle squarely in its path, such as a 

 grain of sand too firmly placed to be moved aside or pushed 

 ahead, it will often come to rest for a moment, back away a 

 short distance and return on the same or on a slightly differ- 

 ent track. This mancEUvre may be repeated a number of 

 times, until perhaps the sand grain is no longer squarely 

 "head-on," but partly to one side and in contact with but 

 one of the frustules, at a corner of the group. The whole 

 group will now gradually tilt itself on its thin edge, draw in 

 against, and seem to grasp, the sand with that one of its own 



* Proceedings, Vol. II, p. 128 et seq. ; Vol. Ill, p. 70 et seq. 



