The polyzoa can neither hear nor see, at least as far as we are able to ascertain, but the delicacy of their sense of touch is very great. "When left undisturbed in a glass of fresh sea-water," says Dr. Johnston,[R] "they push their tentacula beyond the mouth of the cell by straightening the body, and then expanding them in the form of a funnel or bell, they will often remain quiet and apparently immovable for a long time, presenting a very pretty and most interesting object to an observer of the 'minims of nature.' If, however, the water is agitated, they withdraw on the instant, probably by aid of the posterior ligament or muscle; the hinder part of the body is pushed aside up the cell, the whole is sunk deeper, and by this means the tentacula, gathered into a close column, are brought within the cell, the aperture of which is shut by the same series of actions. The polyzoa of the same polyzoary often protrude their thousand heads at the same time, or in quick but irregular succession, and retire simultaneously, or nearly so, but at other times I have often witnessed a few only to venture on the display of their glories, the rest remaining concealed, and if, when many are expanded, one is singled out and touched with a sharp instrument, it alone feels the injury, and retires, without any others being conscious of the danger, or of the hurt inflicted on their mate. The polyzoa propagate by gemmation and by ova or eggs, which, germinating on the inner surface, escape at a later period into the visceral cavity, and are finally discharged into the wide sea, so to fulfil their mission in creation, and people the shores of every clime with myriads of busy workers in horn and in lime, which, with subtle chemistry, they draw from a fluid quarry and build up in textures of admirable beauty and heaven-ordered designs." [Footnote R: "History of the British Zoophytes," 2nd edit. vol. i. p. 259.]