Many of the protists are very sensitive to electric currents, as Max Verworn especially proved by a series of beautiful experiments. Most of the ciliated infusoria and many of the rhizopods (_amœba_) are cathodically sensitive or negatively galvanotactic. When we send a constant electric current through a drop of water in which thousands of _paramœcium_ are moving about, all the infusoria swim at once, with the anterior pole of the body foremost, towards the cathode or negative pole; they accumulate about it in great crowds. If the direction of the current is now changed, the whole swarm at once make in the opposite direction for the new cathode. Most of the flagellate infusoria do just the reverse; they are anodically sensitive or positively galvanotactic. In a drop of water, in which swarms of _polytoma_ are moving about, all the cells swim at once towards the anode or positive pole, when an electric current is sent through. The opposite galvanotropic behavior of these two groups of infusoria in a drop of water, in which they are mixed together, is very interesting; as soon as a constant stream enters it, the ciliata fly to the cathode and the flagellata to the anode. When the current is reversed the two swarms rush at each other like hostile armies, cross in the middle of the drop, and gather at the opposite poles. These and other phenomena of galvanic sensation show clearly that the living plasm is subject to the same physical laws as the water that is decomposed into hydrogen and oxygen by an electric current. Both elements _feel_ the opposite electricities. SCALE OF SENSATION AND IRRITABILITY 1st Stage: SENSATION OF ATOMS. Affinity of the elements in every chemical combination. 2d Stage: SENSATION OF MOLECULES (groups of atoms): in the attraction and repulsion of molecules (positive and negative electricity, etc.). 3d Stage: SENSATION OF PLASTIDULES (micella, biogens, or plasma-molecules): in the simplest vital process of the monera (chromacea and bacteria). 4th Stage: SENSATION OF CELLS: irritability of the unicellular protists (protophyta and protozoa): erotic chemotropism connected with the nucleus and trophic with the cell-body. 5th Stage: SENSATION OF CŒNOBIA (volvox, magosphæra). With the formation of cell-communities we have association of sensations (individual feeling on the part of the social cells together with common feeling on the part of the community). 6th Stage: SENSATION OF THE LOWER PLANTS. In the metaphyta or tissue-plants all the cells are still equally sensitive at the lower stages: there are no special sense-organs. 7th Stage: SENSATION OF THE HIGHER PLANTS. In the higher metaphyta specially sensitive cells, or groups of cells, with a specific energy, are developed at certain points: sense-organs. 8th Stage: SENSATION OF THE LOWER METAZOA, without differentiated nerves or sense-organs. Lower cœlenteria: sponges, polyps, platodaria.