MKMOIHES. 9 



stipes is concurrent with that of the silex, or siliceous 

 skeleton of thc Diatom. Be this as it may, it is an 

 undoubted offset of the frustule, and fpom which the 

 frustnle can detach itselfatany moment after it lias 

 fulfilled its purpose in the process of development. 



In the genus Cocconema, the stipes are of a compa- 

 rative^' large size; notwithstanding this, from their great 

 transpareney there is much diffîculty in demonstrating 

 their présence; and the attennated contractile filament 

 of even larger frustnles can in no way he seen. When 

 high powers are employed, it is manifest to the prac- 

 ticed eye, that the rapid movements performed by the 

 free valves of Diatomaeea? are not to be explained upon 

 any endosmotic and exosmotic theory. If taken at a period 

 of the year favourable to the conditions of Diatom life, 

 they will be seen to move aboutwith considérable rapi- 

 dity over the field of the microscope often moving against 

 the current of the water in which they exist. They will 

 avoid organisms, or impediments lying in their way and 

 separate themselves from a mass of minerai or vegetable 

 matter. Motivity of this nature is certainly deserving of 

 a betler description than that giventoit by someauthors : 

 « a languid roll » ; and which only occnrs when the 

 frustule is either dying, or in a state of exhaustion from 

 want of oxygen. 



Before l proceed fnrther, it will be as well to notice, 

 en passant, other théories of thc movements of Diatoms. 

 First, that of an undulatiog protoplasmic membrane; 

 second, that ofciliaarrangcd in numerical ordcr through- 

 ont the suturai Une of the frustule; third, cilia project- 

 iug from the openings, « foramina », at the extremities 

 of the frustule. Nâgeli's modified endosmose and exos- 



