174 ROMANCE OF LOW LIFE AMONGST PLANTS. 



when they cannot pass above or below them. Some- 

 times, when they are entangled in a mass of dead 

 organic matter, they put it in motion by their 

 struggles to extricate themselves. You may there- 

 fore consider as certain all that I tell you about the 

 spontaneous motions of our Navicida, 

 which I scarcely regard as a vegetable 

 (Fig. 30)." 



Professor Leidy observes, independ- 

 ently, and apparently with no hypothesis 

 in his mind, that " he encountered a small 

 species of Navicnla, very active, in a rain 

 pool, and found it could move grains of 

 sand, as much as twenty-five times its own 

 V,aa ^wmi^es. supcrfieial area, and probably fifty times its 

 own bulk and weight, or perhaps more." ^ 

 The movements in some species are very peculiar, 

 and those of Bacillaria paradoxus are well known, 

 and often cited. Mr. Thwaites has thus described 

 them : " When the filaments have been detached 

 from the plants to which they adhere, a remarkable 

 motion is seen to commence in them. The first in- 

 dication of this consists in a slight movement of a 

 terminal frustule, which begins to slide lengthwise 

 over its contiguous frustule ; the second acts simul- 

 taneously in a similar manner with regard to the 

 third, and so on throughout the whole filament, the 

 same action having been going on at the same time 

 at both ends of the filament, but in opposite direc- 

 tions. The central frustule thus appears to remain 



* Proceedings of Academy oj Nat, Sciences, PhiladelpJtia (1S75), 

 p. 113. 



