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In the starfishes it is most difficult to secure any digestive 

 juice. Since, as mentioned before, I did not succeed in feeding 

 these animals artificially, I had to be content with a drop of 

 liquid eventually falling out of the organ to be tested. Consi- 

 dering the roughness of such procedure, it is rather remarkable 

 that even these drops had a PH very much different from that 

 of the perivisceral fluid by which, though the utmost care was 

 taken to avoid this, it might have been contaminated. The PH 

 of the radial sac fluid appeared to be 7.3; that of the contents 

 of the stomach 7.1, 7.6 and 7.7 in three samples. 



One fact was very striking in all tests. After the color 

 comparison had been made, the apparatus was frequently left 

 at the same place for some time, before it was cleaned. Then 

 a comparison of the colors would have given entirely different 

 results : the digestive juices which at first were rather far on 

 the acid side of the sea water, more and more turned over 

 to alcalinity. I do not know definitively how to explain this 

 remarkable phenomenon, since the standards were exposed to 

 the same amount of evaporation etc. Maybe it is due to a 

 loss of CO 2 which might be the cause of acidity, since phos- 

 phates are not present in quantities large enough to account 

 for the acidity, as we will see in the next chapter. Gas-bubbles 

 present in the intestine of Tenebrio, have been supposed by 

 Biedermann 8) to consist of CO 2 . 



It will be noted that all these PH s are on the acid side of 

 that of sea-water, which in W o o d s Hole appeared to have 

 a PH of 8.2 8.3. In the experiments of Roaf mentioned above 

 we have seen the same thing in his experiments with neutral- 

 red. For such acidity there must be an explanation and in the 

 next chapter we will endeavor to find such reason. 



First I want to mention another interesting phenomenon, i.e. 

 the relatively high acidity of the true intestine, just behind the 

 stomach in Thyone. In Stichopus we even find real acidity 

 according to Crozier (on p. 388 of 18)), where he says : ,,The 

 yellow fluid, contained in the stomach of an ,,empty" Stichopus 

 gives with indicators an apparent acidity of 5.0 6.5. Fluid 

 obtained by centrifuging the stomach contents of animals engaged 

 in feeding showed acidities varying from 4.85.5, the latter 

 being the most common." Here of course, we have to do with 

 a specific adaptation to the calcareous sand, taken as food by 

 these animals, which as we pointed out in the chapter on the 

 biology of our group, may have considerable economical im- 

 portance, because the calcium present in nature in undissolved 

 form, may be brought into solution in this way. Rain-water 

 present everywhere in nature as dissolving medium, only has 

 a PH of 6.0; the water dripping from the tip of stalactites 

 one of 7.98.0 (Crozier). 



But something of the same nature is also the case in our 



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