DIATOMACE2E. . 309 



rapidity of motion was almost continuous. Not " a series 

 of jerks," nor " a languid roll," but in a lively movement, 

 constantly changing about in search of food, pushing aside 

 obstacles that impeded their course, or, if too large, passing 

 over or under them with the motile force of " beings 

 possessed of a power of self-direction." Fig. 178 was 

 drawn from specimens lately obtained from a pond at 

 Woodford, Essex. 



Siebold says : " I have been unable to detect six open- 

 ings, fig. 180, in the Navicula ; and precisely at the spots 

 which Ehrenberg and .,,- 



others suppose they have 

 seen openings, the sili- 

 ceous cell- membrane be- 

 comes thickened, and 

 forms rounded eminences 

 which project internally. 

 On the same two surfaces 

 upon which the thicken- 

 ings of the siliceous shield 

 of Navicula are placed, Fig. iso. 



there may be observed four lines passing along the middle 

 of the surfaces from one thickening to another. These lines 

 which have been long known, but hitherto little noticed 

 are to be referred to a suture, fissure, or rather gap, in 

 which no siliceous matter is deposited ; so that in these 

 places the delicate primordial membrane which lines the 

 siliceous shield can be brought in close relation with the 

 outer world. It is exactly at these four sutures or fissures 

 that the water surrounding the Naviculce is set in motion. 

 The existence of this current is readily demonstrated if 

 some minute solid particles be added to the water in 

 which are some fresh Naviculce. When water coloured 

 with indigo has come to a state of rest on the object-glass, 

 it will soon be perceived by the microscope that those 

 particles which come in contact with the living Naviculce 

 are set into a quivering motion, though previously quite 

 still. It will also be perceived, that only those particles 

 of indigo are set in motion which are in contact with the 

 four sutures of the siliceous shield ; whilst the particles 

 adherent to the other parts of the shield remain altogether 



