85 



Sitovski 121) in a paper on the biology of the catterpil- 

 lars of the moth Tineola biselliella, has explained the acidity of 

 the contents of their end-gut as due to the presence of uric acid. 

 This does not seem very probable considering the very low 

 degree of dissociation of this acid, but it seemed to me to be 

 desirable to ascertain whether those pa.ts of the gut which 

 excrete uric acid actually have a lower PH- Therefore I made 

 some determinations especially of the rectal coeca of the star- 

 fishes. When we remember that in the cucumbers there is a 

 very decided alcalinity which increases towards the rectum, we 

 must agree that here at least the evidence is entirely negative. 

 But in these rectal coeca some authors have occasionally found 

 a rather strongly acid reaction. I have made some determi- 

 nations for the solution of this question in exactly the same 

 way as described in the chapter on the hydrogen-ion con- 

 centration. To one single drop of the rectal-coeca-preparation 

 described above was added a certain amount of indicator and 

 its color compared with that of a set of standard drops. The follo- 

 wing values were found in different experiments : 7.2 ; 7.4 and 

 7.4. These figures may demonstrate a slightly stronger acidity 

 than that of the stomach contents, but I do not believe that this 

 difference has any significance at all. It would be very dangerous 

 to draw from this conclusions like Sitovski's especially 

 since R o a f occasionally found a strongly alkaline reaction in the 

 liquid squeezed out of the rectal coeca on treatment with NaOH. 



The importance of uric acid as a product of excretion in 

 Echinoderms may be greater than one would at first suppose 

 I refer here especially to some very interesting obser- 

 vations of Mangold 82) on auto-intoxication in sea urchins. 

 The form on which he worked was Arbacia pustulosa a 

 species rather closely related to our A. punctulata. His obser- 

 vations hold equally true for our form, and I would have 

 described these phenomena if he had not done it. 



In Arbacia the faeces, greenish or in our case white, round 

 bodies, are expelled through the cloaca which is situated in 

 the middle of the periproct. The spines show an avoiding reflex 

 first described by von Uexkull as soon as one of 

 these little bodies comes in contact with them. In this way and 

 by the aid of the trifoliate pedicellariae the bodies are carried 

 over and dropped down gradually. In nature, of course, the 

 waves promptly remove them, so that here this complicated system 

 of remo al is of not much use. 



Arbacia is a rather weak form and soon dies in quiet 

 water. This is doubtless due to a toxic action of the faeces 

 on the skin. Even in a well aerated aquarium the animals 

 because the removing force of the waves is lacking. Irritability 



