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1. The fleshy part of Difflugia pyriformis and Difflugia acuminata is in structure much like Amœba proteus. There is a thin elastic surface membrane (plasmalemma), a central fluid mass (plasmasol), containing a large granular nucleus, and a relatively solid layer (plasmagel) which surrounds the plasmasol. Probably there is also a hyaline fluid layer between the plasmagel and the plasmalemma but this, if it is present. is much thinner and less conspicuous than in Amœba.2. Locomotion in Difflugia is normally brought about by the extension of pseudopods, one after another, and attachment to the substratum at the tip, followed by contraction which pulls the shell containing the body forward. Movement is consequently intermittent.3. The elastic strength of the plasmagel is lowest at the tip of the pseudopods. This results in contraction of the plasmagel elsewhere, and this contraction forces the plasmasol out through the plasmagel tube, causing expansion at the tip of the pseudopod. The plasmasol which is in c...