Cuap. 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 +5, of an inch (203 mm.) in 
length, and weighing only ;;4,, 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, 
18 
