Many free-swimming rotifers practise letisimulation when disturbed or when threatened by what they consider impending danger. If a "pitcher rotifer" (_Brachionus urceolaris_) be approached with a needle point, it will cease all motion and sink; the same is true of the "skeleton rotifer" (_Dinocharis pocillum_) and numerous others of this large family. Again, if a bit of alga on which there is a colony of "bell animalcules" (_Vorticellæ_) be placed in a live box and then be examined with a moderate power, they can be seen to feign death. The rapidly vibrating cilia which surround the margin of the "bells" give rise to currents in the water which can be easily made out as they sweep floating particles toward the creatures' mouths and stomachs. If the table on which the microscope rests be rapped with the knuckles, the colony will disappear as if by magic. Now, what has become of it? If the microscope be readjusted, a group of tubercles will be observed on the alga; these are the vorticellæ. They have simply coiled themselves upon their slender stems, have drawn in their cilia, and are feigning death. In a few seconds one, and then another, will erect its stem; finally, the entire colony will "come to life" and resume feeding until they are again frightened, when they will at once resort to letisimulation. Death-feigners are found in four divisions of animal life; viz., among insects, birds, mammals, and reptiles. Indeed, the most gifted letisimulants in the entire animal kingdom are to be observed in the great snake family. The so-called "black viper" of the middle United States is the most accomplished death-feigner that I have ever seen; its make-believe death struggles, in which it writhes and twists in seeming agony and finally turns upon its back and assumes _rigor mortis_, cannot be surpassed by any actor "on the boards" in point of pantomimic excellence. I do not know of any fish which has acquired this strategic habit, but the evidence is not all in, and some day, perhaps, death-feigners may be found even among fishes.[111]