30 



11. HYDROGEN-ION CONCENTRATION 

 OF THE GUT CONTENTS. 



The enormous importance of the concentration of the 

 hydrogen-ions in almost every biological process, so clearly 

 realised by the numerous investigators who followed the 

 example of Sorensen, justifies a closer investigation of the 

 digestive liquid of the Echinoderms in this respect and a 

 comparison with the very scarce studies on other invertebrates. 



One reason why these studies have always been so very 

 scarce, is the fact that methodologically biologists were so very 

 poorly equipped. It took a long time before the PH work 

 penetrated through all the branches of biology, so that allthough 

 So r en sen's work appeared already in 1902, even biologists 

 with the physiological attitude, have been going on using the 

 old fashioned methods, chiefly the mere coloration of indicators, 

 for the determination of acidities of media to be investigated. 



Plateau (Mem. de 1'Acad. Roy. de Belgique t. 41. 1874) 

 studied a series of myriapods, arachnids and crustaceans as to 

 the reaction of their digestive juices in this way. He found 

 that in all articulates the gut contents are alcaline, sometimes 

 near neutrality, but never acid. In a later paper he states that 

 frequently a weakly acid reaction is found in carnivores and 

 omnivores, but that the digestive juice of herbivorous species 

 is always alcaline. 



Other authors came to similar results, mostly the contents 

 of the fore-gut are more acid than those of the other parts. 

 On this point I will come back presently. 



The only investigator who ever studied the reaction in 

 Echinoderm guts, was Roaf 108). Though his method was 

 relatively up to date in his days, it does not give us as 

 complete information as one might desire. The argument of 

 his investigation was the question which we have discussed in 

 the chapter on the enzymes, whether it is possible to have a 

 tryptic and a peptic enzyme present, working at the same 

 time. He succeeded in accomplishing what I never could get 

 done, his starfishes and urchins took fibrin colored with diffe- 

 rent indicators. He now determined at the changing point of 

 which indicator the PH of the intestinal juice lies. The follo- 

 wing indicators were chiefly used : dimethyl-amido-azo-benzene, 

 congo-red, litmus, neutral-red, phenolphthalein. 



In the case of the sea-urchin whose feeding reactions this 

 author describes in detail, he found by comparing the results 

 obtained with different indicators, that the hydrogen-ion con- 

 centration of their intestinal juice is about that of the changing 

 point of neutral red, i.e. PH 8. This indicator which is 

 colored yellow by sea water, turns red in the gut of the urchins. 

 The intestinal juice is alcaline to litmus however. 



