Chap. 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 ttAu^ 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 



