Cuap. XI. GENERAL SUMMARY. 265 
process often is the formation of a dark red, bag- 
like mass of protoplasm, which afterwards divides 
and undergoes the usual repeated changes of form. 
Before any aggregation has been excited, a sheet of 
colourless protoplasm, including granules (the prim- 
ordial utricle of Mohl), flows round the walls of the 
cells; and this becomes more distinct after the con- 
tents have been partially aggregated into spheres 
or bag-like masses. But after a time the granules 
are drawn towards the central masses and unite with 
them; and then the circulating sheet can no longer 
be distinguished, but there is still a current of trans- 
parent fluid within the cells. 
Aggregation is excited by almost all the stimulants 
which induce movement; such as the glands being 
touched two or three times, the pressure of minute 
inorganic particles, the absorption of various fluids, 
even long immersion in distilled water, exosmose, and 
heat. Of the many stimulants tried, carbonate of 
ammonia is the most energetic and acts the quickest: 
a dose of tys;a5 Of a grain (00048 mg.) given to 
a single gland suffices to cause in one hour well- 
marked aggregation in the upper cells of the tentacle. 
The process goes on only as long as the protoplasm 
is in a living, vigorous, and oxygenated condition. 
The result is in all respects exactly the same, 
whether a gland has been excited directly, or has 
received an influence from other and distant glands. 
But there is one important difference: when the 
central glands are irritated, they transmit centri- 
fugally an influence up the pedicels of the exterior 
tentacles to their glands; but the actual process of 
aggregation travels centripetally, from the glands of 
the exterior tentacles down their pedicels. The ex- 
citing influence, therefore, which is transmitted from 
