5 Trot. 
PROTOZOA. 
without apparent addition of a sensation. 2. By modification of the 
sensation of necessity for breathing, owing to modification of the inter- 
change of gases. 3. By setting up a specific process, which probably 
answers to our sensation of light. Of the foregoing, the first mode may 
occur either alone, or in combination with the second, or there may be a 
combination of the first and third. The motions of the Oscillarinece and 
mobile Diatomacea, such as Navicula and Pinnularia , were determined 
to be referable to the first category, movement being here intimately 
connected with the presence of free oxygen, which, if not present, can be 
produced by these organisms in the light. When movements ceased through 
the absence of light, it was found that they might be revived by expos- 
ing the diatoms to the red band of the spectrum, the space between the 
lines b and c promoting the greatest activity. The ciliate Infusorian 
Paramcecium bursarium , is quoted as an example of the second order. Of 
this type it was observed that when the proportion of oxygen is normal, 
or somewhat in excess, the Infusorian is usually quiescent ; if, however, 
the supply sinks below this degree, the animalcule becomes restless and 
moves to places in which there is more oxygen, such as the edges of the 
cover-glass ; in good light, but under otherwise similar conditions, the speci- 
mens distribute themselves equally throughout the drop ; active swim- 
ming is the consequence of serious diminution of the oxygen ; if strong 
light is then applied for some minutes, the Paramcecia course rapidly 
about, and if insufficient supplies of oxygen are added from without, they 
show themselves very sensitive to alterations in the illumination of the 
spectrum, prefering more particularly, as in the preceding case, the red 
band between the lines b and c. It is maintained, as an explanation of the 
foregoing phenomena, that, in default of oxygen from without, the 
chlorophyll contained in the mesoplasm of the liviug Paramcecium acts as 
it does in plants, viz., excretes oxygen, and exhibits in its action the same 
dependence on amount and quality of light as do the movements of the 
animalcules. In the Flagellate form, Euglena viridis, which with its 
allies Colacium and Trachelomonas , are taken as types of the third order, 
the tension of oxygen has little to do with the movements ; in dark- 
ness and great dearth of oxygen, gradual dissolution produces an increas- 
ingly feeble sensitiveness to light, but even under high tension of oxygen 
the reaction with light appears to be always less than usual ; when the 
drop of water is partially illuminated the Euglerae gradually assemble in 
the lighted area, aud usually remain there ; if a shadow is thrown upon 
the anterior chlorophyll-less portion of the body, the animalcule turns and 
behaves as though wholty in darkness; this, it is maintained, is not due 
to the eye-spot which is placed here, as the reaction is effected when the 
darkness first reaches the pootoplasm outside it ; a like sensitiveness of 
the anterior end of the body is generally distributed amongst animals, 
occurring in Paramcecium bursarium , in spite of the greater amount of 
chlorophyll contained in the posterior region. The difference between 
Paramcecium and Euglena in relation to light, is more distinctly shown by 
the use of the spectroscope ; while the former prefers the slightly 
refrangible red rays, the latter prefers the blue end of the spectrum, 
whether gaslight or daylight be employed. 
