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Universitf/ of California Puhlications hi Zoology [Vol. 20 



These organisms show remarkable mobility when observed under 

 conditions as nearly normal as possible. I have attempted to depict 

 something of this plasticity in figure K. As they travel amid the 

 debris in the intestinal contents, which has been removed with 

 them, a tendency to penetrate is much more noticeable than any 

 avoiding reaction. Instead of reversing the ciliary action, backing 

 away and taking a new direction as would paramaecia, the balantidia, 

 when they come in contact with a solid object, rather apply themselves 

 to the surface, round up, and seem to roll along it. After a moment 

 of such slow contortion, they may swim away in a new direction. 



Fig. K. Diagrammatic illustration of the plasticity of the organism, resulting 

 in ability to pass through remarkably small openings. 



determined b.y the direction of the anterior end. They avail them- 

 selves of the slightest opportunity to force their way through or 

 between any obstacles. The anterior end, especially the thickened 

 eetoplasmic portion, becomes at such times decidedly elongate and 

 conical (fig. K, &). The cilia of this region beat spirally producing a 

 boring action as this anterior tip is protruded into any slight opening. 

 This action has in many instances been observed to cause two obstacles, 

 either of which was larger than the organism itself, to separate 

 sufficiently to allow it to pass between. The aperture need not be 

 one-half of the diameter of the animal for the latter will constrict 

 (fig. K, c) and the fluid contents flow through anteriorly as it 

 progresses, resembling the process of putting a bag of beans through 

 a small hole in a board. Throughout observations of the activity of 

 these organisms, one is impressed with their fitness for penetrating 

 the mucous lining of the intestine and the underlying tissues. Its 

 thigmotropic response, its boi'ing action, and its extreme plasticity, 

 all seem to be adaptations for the function of penetration. 



Ectoplasm. — Immediately underneath the pellicle, the cytoplasm 

 is differentiated to form the ectoplasm {cct., fig. I; pi. 27, figs. 1-7). 

 The eetoplasmic layer, except at the anterior end, does not exceed two 



