A very suitable object among rhizopods for the study of conductivity, and which is everywhere easily procured, is _Difflugia_. This species living in small pools has a delicate urn-shaped, pear-shaped or flask-shaped capsule built up of sand grains, diatomes or material produced by the organism itself. From the opening the protoplasm extends often to a considerable length its finger-shaped hyaline pseudopods. When _Difflugia_ is placed in a flat dish in water and observed under the microscope, it is frequently seen to extend from the opening long pseudopods in exactly opposite directions, which reach for a considerable distance on the bottom. These offer particularly favorable conditions for the study of the conduction of excitation. When this animal is placed under a microscope, the pseudopods are very readily stimulated at any position to a desired extent by means of a sharp needle, to which fat has been previously applied and subsequently the excess removed. The extension of the response from one point toward the other can then be followed with great ease. The pseudopod of the rhizopod has the great advantage over the nerve that its excitation can be directly observed. The excitation following weaker stimulation is manifested by a wrinkling of the previously completely smooth surface; stronger stimulation produces differentiation of the hyaline protoplasm to a strongly refractive strand in the axis and a turbid myelinlike mass at the periphery, the pseudopod at the same time retracting toward the central cell body. In spite of all these occurrences being of microscopic dimensions, still with some practice it is quite possible to experiment on them under the microscope. In this way I found it comparatively simple to study the fundamental principles of conductivity.[80] [80] _Max Verworn_: “Psycho-physiologische Protistenstudien. Experimentelle Untersuchungen.” Jena 1889. [Illustration: Fig. 15. _Difflugia urceolata._ A--Weak local stimulation at the end of a long extended pseudopod. B--Stronger local stimulation applied to the end of a long pseudopod.] [Illustration: Fig. 16. _Difflugia urceolata._ A--In non-stimulated condition. B--The same individual locally stimulated in the middle of a long extended pseudopod. The excitation spreads in both directions, centripetal as well as centrifugal. ]