86 



and capacity for reaction disappear, the skin shows traces of 

 decay and the spines fall off one after another. 



These phenomena can be prevented by regularly cleaning 

 the animal with sea-water. After the faeces had worked 

 for one hour, this recovery, according to Mangold, could 

 no longer be obtained. 



This doubtless proves that a toxic action of some kind is exerted 

 on the animal by its own faeces. Since faeces of other aminals do 

 not cause the same phenomena, we can not have to do with 

 a mechanical lesion, but a substance must act here, „deren 

 physiologische Wirkung auf die lebende Arbacia der Wirkung 

 von Sauren (! v. d. H.) gleicht" (Mangold). 



„Ueber die Natur dieses Stoffes etwas bestimmtes aus zu 

 sagen, ist vorerst nicht moglich, die Frage wird sich aber jeden- 

 falls erledigen miissen, wenn einmal die Verdauung der Echino- 

 dermen iiberhaupt die langerwiinschte Bearbeitung gefunden hat." 



It seems probable that these phenomena may be due to 

 the presence of uric acid. Isolated pieces of shell, with the 

 spines on them, live much longer than the intact animal, even 

 when no care is taken to change the water. Spines and pedi- 

 cellariae keep alive easily for four days on such pieces. The 

 following observation is interesting in this connection: A 

 specimen of Arbacia was brought into the laboratory in a 

 badly injured condition. Almost half of the shell, including the 

 periproct, was lacking. This piece of animal remained alive 

 for 5—6 days, while normal animals frequently already begin 

 to show loss of spines and other symptoms of bad health 

 in less time. It is a beautiful illustration of the autonomy 

 of the different parts of the urchins body, of a coordinated 

 functioning of this „republic of reflexes" without central impulse, 

 on the other hand it shows that the symptoms observed in the 

 intact animals are due, if not exclusively, yet chiefly to the 

 accumulation of faeces on their body. 



Something of the same nature has been observed by vonUexkull, who 

 saw brittle stars moving around for days after the major part of the disc, 

 including the stomach, had been removed. 



Arbacia seems to be much more liable than any other genus 

 to injure itself by its own faeces. This might be due to two 

 of its peculiarities : 1 . the spines of this species converge toward 

 any stimulated point (von Uexkull) and in that way will 

 prevent the faeces from falling down, 2. the ectoderm is devoid of 

 cilia so that the faeces are not swept off by ciliary action. These 

 forms are largely dependent on the washing produced by the 

 ripples on the shore. 



The substance which von Uexkiill postulates as se-(ex-)- 

 cretion of the skin of the Echinoderms, may be identical with this 

 uric acid. The pedicellariae of the starfishes are known to take 

 hold of every strange object whatever it may be and to hold 



