INFUSORIAL ANIMALCULES. 149 



summer of 1854, I frequently noticed in many of the more common- 

 met-with forms of the Diatomaceae a similar arrangement of cilia. I 

 have attentively watched a diatomean moving slowly across the field 

 of the microscope ; when upon meeting with an obstacle to its progress, 

 it has changed its course, or pushed the obstacle aside, as if conscious 

 of an impediment. I have again and again satisfied myself that their 

 motive power is derived from cilia, so arranged at either end, in some 

 apparently around central openings, that they might readily act as 

 propellers, or paddles. This arrangement is merely indicated in the 

 very rough sketch I made at the time, hoping to have had other op- 

 portunities of rendering my illustra- 

 tions more perfect. Before satisfying 

 myself of the presence of cilia, I thought 

 the motion of these little creatures 

 somewhat remarkable, steering their 

 course as they did by a power which 

 they evidently were able to call into 

 action, or restrain at will. I was there- 

 fore agreeably surprised to find this 

 motive power due to cilia. The distri- 

 bution of the cilia differs from that observed in the Desmidiacece ; the 

 ciliary motion seen in which I believe to be due to a physical force 

 acting independently of any controlling power. On the contrary, with 

 the Diatomacese, their cilia may be said to act in obedience to a will, 

 for intervals of rest and motion are clearly perceptible. 



With Kiitzing, we may say that every diatomean is formed of a 

 siliceous shield, and a soft substance therein contained ; this shield 

 consists of pure silica, or in some cases, perhaps, of silica combined 

 with alumina. Nageli believes the silica is deposited in the outside 

 organic membrane, but this he believes to be of a vegetable nature. 

 In fact, an organic membrane ought to exist ; for the silica could not 

 become solid, except by crystallising or depositing itself on some pre- 

 existing substance. On the other hand, we cannot admit, with Nageli, 

 that it has been deposited externally ; for in many genera, and espe- 

 cially in the Achnanthidia, the siliceous shield is covered with a very 

 delicate dilatable membrane, itself containing silica, as is proved by 

 its sustaining unchanged the action of fire and acids. Therefore, com- 

 paring this shield with other organic formations, whether animal or 

 vegetable, containing, in like manner, either silica or some other so- 

 called mineral element, we might reasonably consider it to be formed 

 of an organic tissue permeated by silica. 



