31 



The indicator diffused out of the food masses, so that no 

 definite information could be obtained as to the reaction of 

 the middle- and end-gut. 



Asterias was fed on fibrin which had been colored with 

 congo-red. The initial reaction proved to be on the alcaline 

 side of this indicator, that is PH ) 4. The rectal coeca are 

 blue from the beginning and remain blue. This is interesting 

 in connection with their function as uric acid excreting organs, 

 which will be discussed in chapter 23. On treatment with 

 alcali however they contract and squeeze out a liquid which 

 instantaneously changes the indicator into red. After their 

 initial alcalinity the food masses are transported into the 

 interior of the radial organs, which show a distinctly blue 

 color, f ,as if containing a blue solution". This would mean a 

 very high acidity, which Roaf does not quite seem to realise, 

 since the changing point of congo-red is at PH 4. This 

 acidity has however also been found by other authors. ,,Das 

 Sekret reagiert schwach sauer". Schneider HI), p. 653. ,,Les 

 coecums pyloriques secretent un liquide a reaction acide." D e 1 a g e 

 et Herouard 136). p. 66. Also Stone 124) '). Chapeaux 

 finds the secretion alcaline. 



The same strongly acid reaction was observed in the small 

 species, Porania pulvillus, which was fed on vesuvian-brown 

 (changing point also at PH = 4). Roaf concludes that the 

 reaction of the pyloric coeca is about that of a decimolar 

 solution of sodium di-hydrogen phosphate. Here also the rectal 

 coeca appeared to have a brown color, which shows an acid 

 reaction. 



Of very recent date is a paper of Jameson and Atkins61 a ) 

 on the physiology of the silk-worm. These authors used the more 

 recent method of Clark and Lubs 2 ), see for a description 

 of these methods the excellent book of Clark on the deter- 

 mination of hydrogen-ion concentration 134) , comparing the 

 color of a drop of digestive juice added with some indicator 

 to the colored plates of Clark 134). A decided acidity could 

 be observed in the gut-contents of the imagines, the PH : 5.2 

 5.8. In the larvae however the digestive secretion proved to be 

 alcaline (PH : '- 9.0 9.8) ; in the hind-gut somewhat less strongly 

 alcaline (?H : 8.4). The acid mulberry leaf is apparently ,,over- 

 neutralised" in the intestine. 



My own experiments were made with the same indicator 

 method. I had the privilege of using a set of Sorensen- 

 standards, made up by Dr. }. B. Col lip of the U.o. Toronto 



T ) Stone found the reaction of the secretion of the pyloric coeca slightly 

 acid: they turned litmus blue. This acidity must be due to some organic acid 

 since tropaeolin OOO which is extremely sensitive to mineral acids showed 

 no change of color. Lactic acid? v. d. H. 



2 ) J. Bact. 2. 1917. 1,109,191. 



