84 



2. The secretion is evaporated down, treated with absolute 

 alcohol, then filtered. The residue is taken up in boiling water, 

 then filtered. Typical uric acid crystals result. 



These two' concordant and independent observations make 

 the presence of uric acid probable, if not certain. They show 

 that it may be excreted into diiFerent parts of the digestive tract. 



Whether or not the uric acid is excreted into the radial sacs and passed 

 over to the stomach pouches or excreted into the pouches themselves is 

 not stated by Griffiths. Neither whether these,, livers" are capable of 

 destroying uric acid, like many true livers, especially those of the dogfish 

 (Scyllium catulus) ; see Scaffidi 114). 



The same substance was also found in our Thyone. If the 

 hquid contents of the rectum, which forms a distinct swelling 

 in this species, or of the end parts of the gut are collected and 

 a test is made for uric acid in them, strongly positive evidence 

 is as a rule obtained. The same thing is frequently true of the 

 contents of the middle gut. Here also uric acid seems to be 

 excreted into the intestine. Remarkably enough C o h n h e i m did not 

 obtain a positive murexide-test in the faeces of Holothuria tubulosa. 



From all this evidence I have concluded that uric acid plays 

 a very important role in the meta- or catabolism of the Echi- 

 noderms. It seems to be the solution of the dark problem of 

 excretion in this group. None of the other waste products of 

 the animal machinery could be detected. In our chapter on the 

 perivisceral fluid we mentioned already the absence of ammonia 

 in detectable quantities. Neither does urea seem to be present. 

 Griffiths searched for it in vain in the radial sacs of 

 the starfish which he considered to be organs of excretion. Four 

 tests: 1. the mercuric nitrate test — no white precipitate, 2. the 

 Na-hypochlorite test — no N-bubbles, 3. the absence of urea 

 nitrate after treatment with nitric acid, 4. a negative N e s s 1 e r, 

 all gave negative results which makes the presence of this 

 substance very improbable. Neither are guanin or Ca-phosphate, 

 the regular products of excretion in Cephalopodes and Lamelli- 

 branchiates, present here (compare p. 81 on the Holothurians). 



The gut, especially the end parts, seems therefore to be 

 the organ of excretion in Echinoderms and the most important 

 substance excreted uric acid. 



This method of excretion is by no means rare or strange. In 

 almost all arthropods we find just the same thing and the 

 Malphigian tubes in insects and other groups, seem to be merely 

 parts of the mid-gut, specialised for the purpose of excretion. 

 They are however by no means the only organs with excre- 

 tory function, for the whole mid-gut may be involved in the 

 like duty, as stated by von Gorka 46) on p. 329 of his paper 

 on the physiology of the Malphigian tubes, where he says: 

 „Hieraus folgere ich. dasz auch der Mitteldarm an der Aus- 

 scheidung der Harnsaure teilnehmen kann". 



