COLLECTING AND PRESERVING DIATOMACE^E. 153 



shield of Navicula are placed, there may be observed four lines pass- 

 ing 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 prim- 

 ordial 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 Navicula 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 Naviculae. 

 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 Naviculse are set into 

 a quivering motion, though previously quite still. It will also be per- 

 ceived, that only those particles of indigo are set in motion which are 

 in contact with the four sutures of the siliceous shield ; whilst the par- 

 ticles adherent to the other parts of the shield remain altogether mo- 

 tionless. Another striking motion is perceptible in these particles 

 when they come in contact with the sutures of the siliceous shield, 

 feeing forced rapidly up and down upon it. Those particles which are 

 propelled from the terminal towards the two central eminences, are 

 never observed to pass beyond the latter; at this point there is always 

 a quiet space, from which the particles are again repelled towards the 

 extremities. This proves that the linear sutures do not extend over 

 the central eminences of the shield. At these clefts the current is 

 sometimes so strong, that comparatively large bodies are set in motion 

 by it." 



COLLECTING AND PRESERVING DIATOM ACE m. 



The following directions by the Rev. W. Smith for collecting and 

 preserving the Diatomacese, may prove useful to the student : " Let 

 him provide himself, in the first place, with the necessary apparatus 

 for the field : this includes a good stock of small wide -mouthed bottles, 

 that each gathering may be kept perfectly distinct ; a long rod or 

 stick, to which can be attached a small muslin net ; a cutting-hook 

 of about three inches in length, and a broad flat spoon ; the first, to 

 collect such specimens as float upon the surface, or are held in sus- 

 pension by the water ; the second, to remove the larger algae which 

 may be covered with parasitic Diatorus ; and the third, to skim the 

 surface of the mud for those which lie at the bottom of the pool. 



He will probably find, notwithstanding every care, that his speci- 



