CHAP. III. THE PROCESS OF AGGREGATION. 39 



If a tentacle is examined some hours after the gland 

 has been excited by repeated touches, or by an in- 

 organic or organic particle placed on it, or by the 

 absorption of certain fluids, it presents a wholly 

 changed appearance. The cells, instead of being filled 

 with homogeneous purple fluid, now contain variously 

 shaped masses of purple matter, suspended in a colour- 

 less or almost colourless fluid. The change is so 

 conspicuous that it is visible through a weak lens, 

 and even sometimes by the naked eye ; the tentacles 

 now have a mottled appearance, so that one thus 

 affected can be picked out with ease from all the 

 others. The same result follows if the glands on the 

 disc are irritated in any manner, so that the exterior 

 tentacles become inflected ; for their contents will 

 then be found in an aggregated condition, although 

 their glands have not as yet touched any object. But 

 aggregation may occur independently of inflection, 

 as we shall presently see. By whatever cause the 

 process may have been excited, it commences within 

 the glands, and then travels down the tentacles. It 

 can be observed much more distinctly in the upper 

 cells of the pedicels than within the glands, as these 

 are somewhat opaque. Shortly after the tentacles have 

 re-expanded, the little masses of protoplasm are all 

 redissolved, and the purple fluid within the cells be- 

 comes as homogeneous and transparent as it was at 

 first. The process of redissolution travels upwards 

 from the bases of the tentacles to the glands, and 

 therefore in a reversed direction to that of aggre- 

 gation. Tentacles in an aggregated condition were 

 shown to Prof. Huxley, Dr. Hooker, and Dr. Burdon 

 Sanderson, who observed the changes under the 

 microscope, and were much struck with the whole 

 phenomenon. 

 4 



