For a phylogenetic physiology the Echinoderms are one of 

 the most interesting groups. The great importance they have 

 for general physiology has already been realised clearly by 

 von Uexkiill, Jordan and other authors. Their study throws 

 a light on many burning questions of comparative physiology- 

 not to speak of the role their eggs play in genetics-because one 

 organ has many functions and one function many organs 

 see f. i. the chapter on respiration ; specialisation and inte- 

 gration take place before our eyes. Their absolutely indepen- 

 dent anatomical structure complicates this study still more. 



Many of the older investigators have made more or less 

 serious mistakes. One of those workers who by their work and 

 by the authority they had, have done more evil than good, is 

 Krukenberg. This author who by his astonishing working 

 capacities, has been quoted by a great many authors, worked 

 without a sufficient knowledge of the histology and anatomy 

 of the animals he studied. As an example I may mention one 

 of his experiments in which he calls the well-known yellow 

 genital organs of Cucumaria : liver no gut appendices are 

 found in Holothurians . Starting from this preconcieved idea 

 he finds enzymes in them. He also finds these in the waterlungs, 

 which as we can find in every text-book, are organs of respi- 

 ratory and perhaps excretory function. In a glycerin extract of 

 these organs he finds a peptic enzyme, but no trypsin and no 

 amylase. One page before this statement the same author says 

 emphatically that ,,alle Versuche, aus den Polischen Blasen, den 

 Cuvierschen Organen, den Wasserlungen und dem Blute der 

 Holothurien, Fermente zu extrahieren, nur zu negativen Resul- 

 taten fuhrten". 



He finds proteolytic enzymes in the digestive juice, but is 

 unable to detect them in a glycerin extract of the intestine. 

 He concludes that the enzyme present must be derived from 

 the food, from which it gets free by autodigestion, which is 

 the more improbable because the larger part of the food of 

 the Holothurians are plant-tissues. Remarkable is his result that 

 glandular cells are present in the blood vessels of the Holo- 

 thurians, which secrete the enzymes. He does not try to explain 

 how these enzymes get through the wall of the gut, but the 

 work of Enriques 37) has elucidated this question sufficiently. 



Krukenberg's result that a peptic and a tryptic enzyme are 

 present simultaneously in Echinoderms as in many other groups 

 he studied, will be discussed in chapter 6. a. 



The unreliability of K r u k e n b e r g's work is a pity, because of 

 the great influence he had in his time and because his work 

 contains so many precious data, which are of not much value 

 in that way. 



Another author whose conceptions the present author is not 

 prepared to follow, is Otto Cohnheim. His plan, the study 



