MEMORANDA. 235 



The instruments for which the medals shall have been 

 awarded will be retained by the Society as standards ; and the 

 successful competitors must enter into a guarantee to supply 

 their microscopes at the foregoing prices, and of equal quality 

 with those retained, and to change them if not found satis- 

 factory. 



The Council, in all cases, expressly reserves the power of 

 withholding the premium or medal altogether, should the 

 essays and articles sent in competition not be considered 

 worthy of reward. 



The essays and articles intended for competition must be 

 delivered, addressed to the Secretary, at the Society's house, 

 free of expense, on or before the 1st of May, 1855. 



CiUa in Diatomacese. — During my examinations, near the end 

 of the last summer, of the ciliary motion in the Desmidieae, I 

 frequently noticed in many of the more commonly met with 

 forms of the Diatomaceae a similar arrangement of cilia. I 

 have attentively watched Diatomaceae moving slowly and 

 steadily across the field of the microscope, when upon meet- 

 ing with any obstacle to their progress they have changed 

 their course, or pushed the obstruction aside, as if conscious 

 of an impediment, I have repeatedly satisfied myself that 

 their motive power is derived from cilia, arranged around 

 openings at either end ; in some 

 around central openings, which 

 with those cilia at the ends act 

 as paddles or propellers. This 

 arrangement is indicated merely 

 in the very rough sketch I made 

 at the time, as I then antici- 

 pated other opportunities for 

 the purpose of rendering them 

 more perfect. Before I had 



made out the cilia, I thought it very remarkable to see these 

 little bodies moving along, and steering their course by a 

 power which they were evidently able to call into action and 

 restrain at will, I was therefore agreeably surprised to find 

 this motive power due to cilia. The position assigned to the 

 cilia, it will be observed, differs much from the ciliary pro- 

 cesses found in the Desmidieae, and which is only, I believe, 

 a physical force acting independently of a.ny controlliruj power ; 

 on the contrary, with the Diatomaceae their cilia appear to 

 act in obedience to some will, for intervals of rest and motion 

 are most clearly to be distinguished ; and this knowledge 

 would naturally induce a doubt, or cause one to inquire once 



