Chap. XI GENERAL SUMMARY. 263 



namely from the gland on the summit of the same 

 tentacle, and from one or more glands of the neigh- 

 bouring tentacles. Tentacles, when inflected, re-ex- 

 pand after a time, and during this process the glands 

 secrete less copiously, or become dry. As soon as 

 they begin to secrete again, the tentacles are ready 

 to re-act ; and this may be repeated at least three, 

 probably many more times. 



It was shown in the second chapter that animal sub- 

 stances placed on the discs cause much more prompt 

 and energetic inflection than do inorganic bodies of 

 the same size, or mere mechanical irritation; but 

 there is a still more marked difference in the greater 

 length of time during which the tentacles remain in- 

 flected over bodies yielding soluble and nutritious 

 matter, than over those which do not yield such 

 matter. Extremely minute particles of glass, cinders, 

 hair, thread, precipitated chalk, &c., when placed on 

 the glands of the outer tentacles, cause them to bend. 

 A particle, unless it sinks through the secretion and 

 actually touches the surface of the gland with some 

 one point, does not produce any effect. A little bit 

 of thin human hair xoVo of an inch ('203 mm.) in 

 length, and weighing only y^|4-o of a grain (-000822 

 mg.), though largely supported by the dense secre- 

 tion, suffices to induce movement. It is not probable 

 that the pressure in this case could have amounted 

 to that from the millionth of a grain. Even smaller 

 particles cause a slight movement, as could be seen 

 through a lens. Larger particles than those of which 

 the measurements have been given cause no sensation 

 when placed on the tongue, one of the most sensitive 

 parts of the human body. 



Movement ensues if a gland is momentarily touched 

 three or four times ; but if touched only once or twice. 



