254 PHYSIOLOGY OF THE INVERTED RATA. 



to have increased from the earliest down to the present 

 geological period." 



The Echinodermata. 



We have already alluded to the secretion of the protective 

 skeleton in these animals ; consequently we proceed to 

 describe the excretory organs of the Astcridea, being an im- 

 portant order of the Echinodermata. 



The author * has shown that the five sacs of the stomach 

 of Uraster ruhcns sometimes act as renal organs. With a 

 quantity of the fluid obtained from a large number of star- 

 fishes the following experiments were performed : — 



(i) The clear liquid from these sacs was treated with a hot 

 dilute solution of sodium hydroxide. On the addition of pure 

 hydrochloric acid a slight flaky precipitate was obtained, 

 after standing seven and a half hours. These flakes, when 

 examined beneath the microscope (^ in. obj.) were seen to 

 consist of various crystalline forms, the predominant forms 

 being those of the rhomb. On treating the excretion alone 

 with alcohol, rhombic crystals were deposited which were 

 soluble in water. When treated with nitric acid and then 

 gently heated with ammonia, these crystals yielded reddish- 

 purple murexide crystallised in microscopic prisms. 



(2) Another method was used for testing the fluid contents 

 of the sacs of the stomach of 'Uraster. These fluid contents 

 were boiled in distilled water, and evaporated carefully to 

 dryness. The residue obtained was treated with absolute 

 alcohol and filtered. Boiling water was poured upon the 

 residue, and to the aqueous filtrate an excess of acetic acid 

 was added. After standing some hours, crystals of ttric acid 

 were deposited, and easily recognised by the chemio-micro- 

 scopical tests mentioned above. 



The above-mentioned alcoholic filtrate was tested for urea. 



* See Dr. A. B. Griffiths' paper in Proceelings of Raijal Sociefi/, vol. 44, 

 1'- 325- 



