The hypnoidal state is brief, variable, and unstable. They who have observed the rest-states of the lower metazoa can form a clear idea of the nature as well as of the biological significance of the hypnoidal state in the life of the lower animals. The animal is at rest for a brief period of time as long as it remains undisturbed by external conditions of its environment, or by internal conditions, such as hunger, sexual impulses, or other internal disturbances. Soon the animal begins to move, sluggishly at first, and then more quickly, and if there are no disturbing stimulations, comes to rest, to be again disturbed from its rest-equilibrium by the varying conditions of its environment. The resting state is brief, irregular, differing from the waking state in but slight relaxation, in comparatively slow reactions to stimulations, and in a passive condition of the muscular system. Respiration is regular, and diminished in rate. The heart beat is slightly decreased, and general katabolic activity is somewhat reduced. The animal, however, is quite alive to what is going on. _The animal rests, watching for danger._ Resting and active states alternate periodically, if possible, but usually are irregular. The resting state is but a passive condition in which the animal may be considered to hover between waking and what we describe in the case of the higher animals as sleep. Sleep, in its proper sense, does not exist among the lower representatives of animal life. This state of hovering between waking and sleeping, the characteristic of the hypnoidal state, is no doubt of paramount importance in the life-existence of the lower animals, considering the numerous dangers to which they are continually exposed. The animal must always be on the watch, either for food or for foe. It can only rest or “sleep” with its eyes wide open. The hypnoidal “sleep” can be best characterized as a _subwaking_, “twilight” rest-state. I demonstrated in my experiments that the animal, while in the hypnoidal, subwaking rest-state, is apt to fall into a cataleptic state, especially when the movements are suddenly and forcibly inhibited. This cataleptic state, which reminds one of the hypnotic state, may be observed in the lower animals, such as the frog, the snake, the lobster, the bird, and, to a slighter degree, even in the higher animals, such as the guinea pig, the cat, the dog, especially in the young ones, such as the kitten, the puppy, and the infant.