MÉMOIRES. 15 



spécimens of Pinnularia which présentée! an unusual 

 appearance. Each frustule had two large colourless con- 

 traelile vesicles on either side of the médian nodule, and 

 a central nucleus in each. The outermost, peripheral 

 portion, of the vesicles was retlexed, turned inwards 

 upon itself; the intermediary space hetween the two 

 being occupied hy a fine granular mass. The othea 

 portion of the frustule was packed closely with a rich 

 brownish-yellow coloured endochrome , iuterspersed 

 amongst which ahout the médian line were a few oil 

 globules *. The phenomenon of cyclosis, as I hâve hefore 

 said, was not seeii to take place in any of the many spé- 

 cimens of Diatoms examined. 



Ehrenherg, with objectives inferior to those of the 

 présent day, satisfied himself that the movements of 

 Diatoms were due to a twofold action : that of a retrac- 

 tilc foot, and of retractile ciliary processes. I hâve 

 not heen able to confirm his conclusions. 1 hâve occa- 

 sionally seen bodies closely resembling cilia, sometimes 

 many snch protruding from certain portions of the frus- 

 tules, but I do not believe them to be associated with 

 any external movements. « A retractile foot », or rather 

 an extension of protoplasm beyond the margin of the 

 frustule, as in that of certain Rhizopods, is demonstrable, 

 but it requires considérable inagnification, well managed 

 illumination, and a staining fluid to differentiate tins. 

 In some of the larger frustules I hâve observed a slight 

 protrusion and retraction of an irregularly shaped mass 

 moving in a limited space about the central nodule, also 

 a swarming or rolling motion among the albuminoïd or 



* Drawings of thèse frustules were submitted to the Socictc belge de 

 Microscopic. 



