38 



H. Ludwig reports on the presence of ..clotting" substances 

 in the „ blood" of cucumbers. 



One of the most important characteristics of a blood system 

 is completely absent in Echinoderms. A propulsatory apparatus 

 has never been demonstrated, neither valves, though the axial 

 organ has for a long time been called ..heart". Tiedemann 

 127) was the first to see rhythmical contractions in the blood- 

 vessels of Holothuria; Cohnheim also saw them in those of 

 Cucumaria, but as he says, no definite movement in either 

 direction was seen if one corpuscle was spotted. Enriques 

 37) discusses his observations in detail and comes to the con- 

 clusion that a definite movement actually exists (see the chapter 

 on resorption in Echinoidea and Holothureoidea). 



At any rate, these movements are exceedingly slow and not 

 to be compared to those in vertebrates and higher invertebrates. 



The importance of this blood system in resorption is to be 

 discussed in chapter 20. As far as its respiratory function is 

 concerned, Ludwig in Bronn's Klassen und Ordnungen etc. 

 in the case of the Holothurians is very sceptical on this point. 

 The wonder-nets (retia mirabilia, Wundernetze, reseaux admi- 

 rables) found around that left respiratory tree, are so loosely 

 attached to the lung, the one can scarcely expect them to have 

 any such importance. In the chapter on respiration I hope to 

 demonstrate that the perivisceral fluid actually plays the role 

 of internal respiratory medium, maybe also the water-vascular 

 system. Perhaps it plays a role in excretion ; the excretion by 

 means of phagocytes -after physiological injections and perhaps 

 also in normal conditions- takes place through the water-lungs '). 



The perivisceral fluid is of much more importance; 175 c.c. 

 of the contents of a Sphaerechinus. measuring in toto 225 c.c. 

 is according to Cohnheim perivisceral fluid. Physiologically the 

 contents of the coelomic cavity must play the role of blood, 

 in as far as it is a fluid medium, interposed between intestine 

 and consuming tissue and tissue and organs of excretion. A 

 little closer investigation of the chemistry of this medium shows 

 that apart from the corpuscles which are present in abundance 

 in all groups and which show a phenomenon of clotting, 

 entirely different from that in mammals-yet leading to the same 

 end result (see f. i. the paper of G e d d e s 42) on this point)-, it has 

 practically the same composition as sea water. Cohnheim already 

 found that this is the case. It contains the same percentage of 

 chlorides and the same inorganic constituents. In as far as it 

 is physiologically ..blood" we must investigate if it carries any 

 food- or waste-constituents and whether it plays a role in 

 respiration. The latter topic will be discussed in chapter 24, 

 for the present will test for food and waste constituents. 



^) See the references in the chapter on the respiration. 



