80 



23. EXCRETION IN ECHINODERMS. FUNCTION OF 

 THE RECTAL COECA OF THE ASTEROIDEA. 



About excretion in Echinoderms we are, as far as my know- 

 ledge of the literature goes, only very poorly informed. One 

 thing however is absolutely clear: the representatives of this 

 group do not posses any definite kidney for the elimination of 

 nitrogenous waste. These kidneys, present in most other divisions 

 of the animal kingdom, are, as comparative morphology teaches 

 us, specialised parts of the coelomic epithelium ; in many cases 

 this appears from embryology, in other cases part of the coelomic 

 wall still participates in the excretory activities. Since in the 

 Echinoderms specialised organs have never been demonstrated, 

 the whole coelomic lining apparently must have excretory func- 

 tion. Cells from this peritoneal wall are constantly thrown into 

 the surrounding fluid, where they move about. They are called 

 from the way in which they move, amibocytes and they seem to 

 have a great importance in excretion. Durham calls them sphae- 

 ruliferous corpuscles. Substances like indigo-carmine which in the 

 higher groups, are eliminated by the kidneys are here found to be 

 absorbed eagerly and to be stored by these „nephrocytes ')". Later 

 these cells in starfishes accumulate in the dermal gills, the papulae, 

 in Holothurians in the wall of the waterlungs, where they move 

 to the outside — diapedesis — and degenerate. Sometimes 

 they form large clumps containing the material to be eliminated, 

 as I myself could observe in Stichopus. Part of them also disap- 

 pear through the ambulacral feet. Sometimes the papulae when 

 loaded with these „nephrocytes", are autotomised. 



The picture given above was, in its general lines, first 

 concieved by Durham 34) and 35), also by Chapeaux. Though 

 nobody has ever tried to demonstrate that this method of ehmi- 

 nation which has beer observed in the case of physiological 

 injections — of suspended substances — , is the normal one, 

 it sounds rather probable that some natural waste-products 

 might be eliminated in this way. 



The role which in the starfishes is played by the papulae, is 

 played by the water-lungs in the holothurians. These have 

 been considered to be organs of excretion by a great many 

 authors. Pourtales 102), Oken, Huxley, Bartels 4), 

 Herouard 55), Schultz 116), Bordas 11) and Danielssen 

 and K o r e n 28) all found that migrating cells loaded, either with 

 artificially injected substances (ink etc.) or with normal granules 

 left the coelomic cavity through the waterlungs ^). A beautiful 

 description of this process of elimination has been given by 



') This name has been given by de Ribaucourt. C. R. Biol. 19 ]anv. 



1901. 

 ^) Jourdain 70) also assumes an excretory function for the migrating cells. 



