Caap. X. NATURE OF THE MOTOR IMPULSE. 259 
protoplasm is redissolved at the bending place shortly 
before the tentacles re-expand, showing that the ex- 
citing cause of the aggregating process has then quite 
ceased. Exposure to carbonic acid causes both the 
latter process and the motor impulse to travel very 
slowly down the tentacles. We know that the aggre- 
gating process is delayed in passing through the cell- 
walls, and we have good reason to believe that this 
holds good with the motor impulse; for we can thus 
understand the different rates of its transmission in a 
longitudinal and transverse line across the disc. Under 
a high power the first sign of aggregation is the ap- 
pearance of a cloud, and soon afterwards of extremely 
fine granules, in the homogeneous purple fluid within 
the cells; and this apparently is due to the union of 
molecules of protoplasm. Now it does not seem an 
improbable view that the same tendency—namely for 
the molecules to approach each other—should be com- 
municated to the inner surfaces of the cell-walls which 
ate in contact with the protoplasm; and if so, their 
molecules would approach each other, and the cell-wall 
would contract. 
To this view it may with truth be objected that 
when leaves are immersed in various strong solu- 
tions, or are subjected to a heat of above 130° 
Fahr. (54°4 Cent.), aggregation ensues, but there is 
no movement. Again, various acids and some other 
fluids cause rapid movement, but no aggregation, or 
only of an abnormal nature, or only after a long 
interval of time; but as most of these fluids are more 
or less injurious, they may check or prevent the aggre- 
gating process by injuring or killing the protoplasm. 
There is another and more important difference in the 
two processes: when the glands on the disc are ex- 
cited, they transmit some influence up the surrounding 
