DIATOMACEJE 191 



ditions observed among Chlorophycece in which 

 resting cells are formed by the contraction of the 

 protoplasm and the thickening of the membrane. It 

 is plain that, owing to the firm siliceous membrane, 

 such a course is not open to the Diatoms, and the 

 craticular state the formation of new valves within 

 the old ones is 'the only resource, when a resting 

 condition is called for, to prevent the drying up of 

 the contents. 



The movements of Diatoms have long been a 

 puzzle, and various explanations have been advanced 

 to account for them. This spontaneous movement 

 of the free forms takes place with considerable 

 rapidity and force, and is always backwards and 

 forwards in the direction of their longer axis, 

 sometimes stopping and then going on with a 

 jerking movement, or proceeding more steadily in 

 creeping fashion. This movement is never a free- 

 swimming motion through the water, such as 

 zoospores possess, but is always a movement along 

 some fixed substance with the surface of which the 

 shell is in contact, and it never takes place when the 

 girdle-side is thus in contact. Naegeli ascribed this 

 movement to osmotic currents, while Ehrenberg ex- 

 plained it by the protrusion of cilia or of a pseudo- 

 podium through the so-called suture or raphe (on 

 the valve side). This observation has not been con- 

 firmed, but one proposed by Max Schultze is not 

 unlike it, and appears to be probable enough. He 

 supposes that a small portion of the protoplasm 

 emerges through this suture and effects the move- 

 ment. In favour of this view, which has not, how- 



