SOME PROBLEMS IN THE PHYSIOLOGY OF THE PROTOZOA 297 



than with the Ciliata. Pelomyxa palustris, according to Engelmann 

 ('79), moves energetically in darkness, but if a strong light be sud- 

 denly thrown upon it, a sudden contraction results and the organism 

 rounds out into a spherical mass ; but if light gradually increases, 

 there is no response. Rhumbler ('98) came to a somewhat similar 

 conclusion in regard to Amoeba proteus, holding that food-taking is 

 more energetic in the night than during the day, and is not frequently 

 observed because the intense light of the microscope mirror causes 

 sluggishness. The experiments of Learning and Harrington ('99) 

 upon the effect of light of different colors shows that Amoeba proteus 

 reacts vigorously in red lights, but protoplasmic movements almost 

 cease in rays from the violet end of the spectrum. 



Living protoplasm can also, within a certain limit, accommodate 

 itself to chemical changes in the surrounding medium. Thus, Amoeba 

 may become accustomed to a 4 per cent solution of common salt, 

 if it be slowly added, while a I per cent solution, if added suddenly, 

 will kill them. Heliozoa can be transferred from salt water to fresh, 

 and vice versa. ^Ethalium septicum will live in a 2 per cent solution 

 of sugar if slowly added, and many other instances might be given. 

 Such adaptation on the part of the organism signifies a gradual change 

 in the chemical or physical make-up of its own substance, and in a 

 variety of cases the rapidity of the change varies inversely as the 

 distance from the source, hence as the intensity, of the stimulus. If 

 a stimulus, therefore, comes from a certain direction, the effect, like 

 that of a light stimulus, is a unilateral reaction, to which the general 

 term chemotaxis has been given (Verworn). The greatest variety 

 of substances may give these reactions, water, air, and other gases, 

 alkalies, and acids of various kinds. Pfeffer ('88) found that, in 

 various Flagellidia, including Cryptomonas, several species of Bodo, 

 Monas guttula, 7"repomonas, Polytoma, Englena, etc., different sub- 

 stances cause very different reactions, substances which cause posi- 

 tive reactions in some forms causing negative reactions in others. He 

 also obtained the interesting result that substances which in weak 

 solutions cause a positive reaction, in strong solutions cause a nega- 

 tive one, so that, as in light stimuli, a chemotactic optimum exists 

 toward which the organism constantly strives. In Rhizopoda, 

 although fewer instances of chemotaxis are known, the same general 

 results have been obtained. Stahl's often cited experiments on the 

 mycetozoan ^Ethalium septicum showed that this organism is posi- 

 tively chemotactic toward both harmful and beneficial stuffs. In all 

 cases of harmful action upon the Rhizopoda, the reaction is expressed 

 by the withdrawal of the pseudopodia, rounding out of the body, and 

 final disintegration. With the Ciliata, according to Pfeffer, chemo- 

 tactic reactions appear to be less delicate than with the Mastigophora. 



