ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 83 



Higli tension of oxygen reacts strongly on the movements, for the 

 animals then tend to swim straight or in wide curves away from the 

 point at which oxygen is present in abundance. Strong illumination 

 applied suddenly at this time causes violent movements, and the Para- 

 mcBcium often darts into the darkness, exhibiting the phenomenon of 

 photophobia. Thus this animal is very highly sensitive to differences 

 in the tension of oxygen. 



3. Euglena viridis is taken as the third type. It appears that in 

 both it, Colacium, Traclielomonas, and some allied forms, the tension of 

 oxygen has little to do with the movements. In darkness and great 

 dearth of oxygen gradual dissolution produces, naturally, an increasingly 

 feeble sensitiveness to light ; but even under high tension of oxygen 

 the reaction with light appears to be always less than usual. When 

 the di'op of water is partially illuminated the Euglence gradually 

 assemble in the lighted area, and usually remain there ; if a shadow 

 is thi'own upon the anterior chlorophyll-less portion of the body the 

 animal turns and behaves as if wholly in darkness. This is not due 

 to the eye-spot which is placed here, as the reaction is effected when 

 the darkness first reaches the protoplasm outside it. This sensitive- 

 ness of the anterior end of the body is generally distributed amongst 

 animals, and occurs in Paramcecium bursaria, in spite of the greater 

 amount of chlorophyll contained in the posterior part. The differ- 

 ence between ParamcBcium and EuglencB in relation to light is more 

 distinctly shown by the use of the spectroscope ; whereas the former 

 prefers the slightly refrangible red rays, the latter prefers the blue 

 end of the spectrum, whether gaslight or daylight is employed. The 

 following is a good average sample of the way in which Euglence dis- 

 tribute themselves over the spectrum, and should be compared with 

 the tables given above : — 



Between A and C 3-4ths D (red to orange) . . 2 individuals. 



„ C 3-4ths D and D 5-6ths E (orange to green) „ 



„ D 6-6ths E and & 5-6ths F (green) ..16 



„ h 5-6 ths F and F 4-7ths G (green to blue) 100 „ 



F 4-7ths G and G (blue to indigo) ..24 



„ G and G J H f indigo to violet) .. .. 3 „ 



• In the spectrum the individuals swim in all directions, as in com- 

 plete darkness. The sensitiveness to minute differences in the quality 

 of the light is decidedly greater in the red, yellow, and green than 

 in the blue. Engelmann has not as yet succeeded in finding blind 

 or colour-blind Euglence., but individuals from different localities and 

 in different stages of development often show important variations in 

 their sensitiveness to light. 



G 2 



