This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(176). On cutting off a portion of one of the arms of an Actinia and subjecting it to pressure, it is seen to have, imbedded in the substance of its gelatinous parietes, an immense number of minute organs, now universally known by the name of filiferous capsules. These remarkable structures, which are found to exist very extensively throughout the entire group of polypoid organisms, consist of minute sacculi, wherein may be perceived a slender and highly elastic filament coiled up spirally, but which, on compression, suddenly shoots forth from one extremity of the capsule to a length that is perfectly surprising. It is upon the presence of these filiferous capsules that the adhesive power of the tentacula is supposed to depend; and from the rapidity wherewith prey, when seized, is destroyed by their grasp, it is probable that a poisonous fluid is emitted along with the thread, to the virulence of which its paralysing effects are to be attributed. The presence of these remarkable organs is, however, by no means restricted to the tentacula; on the contrary, they are dispersed over various parts of the body, and exist even in the folds of the ovarian membrane.
In the latter situation, indeed, they are frequently extremely numerous, and comparatively of large size.