SECT. ii. DIATOMACEJE. 203 



soria is still more unprecedented. The frustules, which 

 are narrow, lanceolate, and acute, are joined end to end 

 in a long line by some highly elastic invisible medium. 

 One of the terminal frustules remains at rest while all 

 the others slide over it till the line is so much stretched 

 that they are nearly detached from one another; then 

 they all slide back again in the same manner, and this 

 alternate motion is continued indefinitely at regular 

 intervals of time. The velocity of the diatoms at the 

 free end of the row is very considerable ; in the Bacillaria 

 paradoxa it is ^io^ f an ^ nc ^ 1 ^ n a secon d ; the impetus 

 of one has been observed to upset and even to push 

 aside a plant as much as three times its size which 

 obstructed its path. If the frustule at the free end gets 



Fig. 16. Baciliaria paradoxa. 



entangled, the fixed frustule takes the lead and continues 

 the motion till the other is free. Minute particles in 

 the vicinity are sometimes attracted and dragged after 

 the frustules, sometimes they are repelled, possibly by 

 some invisible organs ; but the whole motion of the 

 diatoms themselves may perhaps be attributed to the 

 action of light and heat upon the highly contractile 

 substance, whatever it may be, which connects their frus- 

 tules, since their motion is exactly in proportion to the 

 quantity of light and heat received, for it ceases during 

 darkness, and is renewed on the return of light ; ulti- 

 mately it may disperse the individual frustules, which 

 are not more than between the i^th and the. fo^th of 

 an inch in length and the 10t ^th of an inch in breadth. 

 This Bacillaria paradoxa (fig. 16) differs from the pre- 



