400 GENERAL PHYSIOLOGY 



body, sometimes almost 2 mm. in size, contains, in addition to a large 

 number of spherical nuclei, innumerable small grains of sand and 

 particles of mud, and hence appears opaque. The movements of 

 Pelomyxa are like those of a sluggish Amosba. The lumpy, proto- 

 plasmic droplet sends out here and there beyond the dark contour 

 of the body and usually in a jerking manner a flat, hyaline pseudo- 

 podium, into which the internal mass streams with its nuclei, sand- 

 grains, etc. As a rule, when the protist is left to itself undisturbed, 

 after some time, as in Amoeba Umax,''- a definite direction is taken, in 

 which alone the protoplasm continues to flow, and the body thus 

 assumes an extended form (Fig. 185, A). But, if, while creeping, 

 the Pelomyxa be stimulated mechanically by jarring, or chemically 

 by the addition of salt solutions, or thermally by warming, it con- 

 tracts immediately and like all naked protoplasmic masses assumes 

 a spherical form (Fig. 185, B). In this peculiar organism Engel- 

 mann (79, 2) discovered a pronounced irritability to light : when the 

 Pelomyxa was creeping in the darkness lazilj' and quietly with its 



A B 



Fig. 185. — Pelomyxa -palustr is. A, Unstimulated, creeping ; B, stimulated, contracted. 



form extended, sudden illumination had the effect that has been 

 spoken above in connection with chemical, mechanical and thermal 

 stimulation. The protoplasmic body contracted suddenly- into a 

 ball, and all motion ceased, to reappear at once, however, upon 

 darkening. Slow increase in the intensity of the light from dark- 

 ness on had no decided influence. The protoplasmic masses of many 

 Myxomycetes, which likewise show contraction-phenomena upon 

 photic stimulation, behave in all respects similarly. 



Engelmann, who has made many discoveries concerning the 

 physiology of the unicellular organisms, found also a peculiar 

 Bacterium; which proved extraordinarily sensitive to photic stimuli. 

 This form, which Engelmann ('83) called Bacterium jJhotomctricum, 

 moves about actively in a drop of water by the stroke of the flagellum 

 which the ends of the body of every motile Bacterium bear. This 

 motion continues only so long as the Baeterium. is exposed to the 

 influence of light. If it be brought into darkness, the motion 

 gradually ceases, and the Bacterium remains still. But, so soon as 

 1 Cf. Fig. 158, p. 367. 



