PROTOPLASM OF PROTOZOA 91 



amoeba results in an increase of volume and so stretches the gelled cyHn- 

 der of protoplasm; the resulting elasticity forces the sol forward where a 

 decrease in volume has occurred upon gelation of the plasmasol. How- 

 ever, in shelled forms, such as Difflug/a (Dellinger, 1906; Mast, 1931a) 

 and Centropyxis, there is an active longitudinal contraction of the plas- 

 magel cylinder which results in locomotion by pulling the shell along. 

 That the circular elastic contraction in Amoeba and the longitudinal 

 active contraction in Dijjiug'm are different mechanisms may be doubted. 

 However, Mast (1931a) has shown that Dijjiugia deprived of their 

 shells move much as does Amoeba. It is to be noted that the source of 

 energy is in the ectoplasm, so that the streaming of the endoplasm in 

 these forms must be of a different nature from cyclosis in other forms 

 such as Paramecium or Frontonia, where protoplasmic streaming is ex- 

 tremely difficult to explain in terms of contraction. 



There are many examples of local contractility in the literature of 

 ameboid movement. Swinging and revolving movements of lobose 

 pseudopods when not in contact with the substrate have been described 

 by Penard (1902), Jennings (1906), Hyman (1917), Kepner and 

 Edwards (1917), and many others. These differential local contractions 

 of the ectoplasm often approximate muscular activity, according to Kep- 

 ner and Edwards. Schaeffer (1926) has described corkscrew-shaped pseu- 

 dopodia in Astramoeba jlageWpoda, with from two to eight spirals which 

 wave about quite like flagella, often making a complete revolution in 

 three seconds. 



The most spectacular instances of local contraction are those in which 

 an Amoeba pinches a large ciliate in half. Mast and Root (1916) de- 

 scribe this process as taking ten seconds for Paramecium and show that 

 it cannot be explained in terms of the surface tension of the Amoeba. 

 Beers (1924) describes the constriction of Frontonia by Amoeba until 

 the former was dumbbell-shaped, and ascribes the pinching to centripetal 

 pressure exercised by an extending collar of protoplasm which pinched 

 the prey in half in eight minutes. Kepner and Whitlock (1921) saw 

 a partly ingested Paramecium constricted much as described by Beers for 

 Frontonia, except for the loss of the cilia from the ingested part. The 

 figures and descriptions of the two latter instances are very similar to 

 those of Grosse-AUermann (1909) for ingestion by invagination in 



