DIATOMACL.E 191 
ditions observed among Chlorophycew 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- 
