§3] EFFECT UPON GENERAL FUNCTIONS 179 



less marked. Among the Myxomycetes, Engelmann ('79) 

 has found that the amceboid Pelomyxa, when suddenly sub- 

 jected to a strong light, contracts into a spherical mass. 

 Sudden darkening or gradual illumination produces no such 

 contraction. Among swarm-spores, Strasbtjegbr ('78, pp. 575, 

 576) has noticed that a sudden diminution of the light puts 

 the quiet Hsematococcus spores again in motion, and makes the 

 Botrydium spores start as though disturbed. Such violent 

 movements of the protoplasm indicate that a very considerable 

 chemical change has taken place in it. 



Passing, next, to the Metazoa, we find that certain smooth 

 muscle fibres are made to contract by the direct action of light ; 

 thus, Steinach ('92) has offered most convincing evidence 

 that the contraction of the iris, in the lower vertebrates at 

 least, may occur as a direct reaction to illumination, even when 

 the eyeball is cut out, and the iris, indeed, separated from 

 connection with the ciliary part of the eye. 



Some of the higher animals react strikingly like Engel- 

 mann's bacteria. Thus, Loeb ('93, p. 103) found that Serpula 

 uncinata retracts into its tube when the hand is passed between 

 it and the light; but sudden increase of illumination has no 

 effect. Nagel ('96, p. 76) finds the same thing in Spirographis, 

 and Andrews ('91, pp. 285, 296) has observed the same phe- 

 nomenon in the eyeless Hydroides dianthus. In these cases 

 the branchias seem to be the sensitive organs. Adult barnacles 

 show a similar sensitiveness to light ; for Pouchet ('72, p. Ill) 

 found that momentary cutting off of the light, as by the shadow 

 of the hand, caused arrest, for several seconds, of the rhythmic 

 movements of protrusion of the appendages from the shell. In 

 this case, the sensitive region has not been located. Some 

 lamellibranchs (Nagel, '96, p. 50) react similarly to increased 

 light. These are examples of a phenomenon which we shall 

 meet with again in considering growth. They serve to show 

 that there is a wide-spread irritability of protoplasm to changes 

 in intensity of light. 



Let us now review the conclusions of this section. Light — 

 especially the thermic rays — is essential to the decomposi- 

 tion of carbon dioxide by chlorophyllaceous plants. The only 

 effective rays are those absorbed by the chlorophyll. The rate 



