96 H. C. VAN DER HEYDE. 



examination of the fluid showed that it was bound to corpuscles. 



These corpuscles are, as far as their appearance under the 

 microscope is concerned, very much like mammalian blood cor- 

 puscles ; they are however not biconcave discs, but seem to be per- 

 fectly round. Their color is the same as that of mammalian cor- 

 puscles. They do not have any active movements as far as could 

 be observed, and under the microscope they have the appearance 

 of perfectly round discs. A nucleus is clearly visible in most 

 cases ; the protoplasm has a granular structure and a cell mem- 

 brane gives a perfectly clear " outline." 



The question arose as to what the function of these corpuscles 

 was. The Polian vesicle forms a part of the water vascular 

 system. In this way they stand on one hand in connection with the 

 ambulacral feet, which are found all over the outside of the body 

 and by means of which the animal moves. This species is radi- 

 ally symmetrical and does not have the so-called bivium. Inside 

 the muscular body wall these podia are represented by their am- 

 pullae and, remarkably enough, all these little ampullse can be seen 

 on diissection to contain the same colored material. On the 

 other hand, the Polian vesicle, through the water vascular system, 

 communicates freely with the ccelomic fluid. The stone-canal 

 stiffened by carbonate of lime, which in other Echinoderms and 

 in larval Holothurians communicates with the sea water, forms 

 here so-called " internal madreporites," one or even five, as in 

 Holothuria tubulosa. This stone-canal is derived from the larval 

 axial sinus, whereas the original pore-canal and external madre- 

 porite disappear. Furthermore the water vascular system stands 

 in connection with the tentacles, the " feelers," which by their 

 active movements may assist greatly in respiration. 



The so-called blood system of the Holothurians consists of a 

 blood ring surrounding the oesophagus and situated inside of the 

 water vascular ring. This system forms a plexus around the gut 

 and water lungs, a dorsal " vessel," and probably stands in con- 

 nection with the coelomic fluid. 



The water lungs are considered to be organs of respiration. A 

 rhythmical contraction o>f the cloacal wall can be seen, resulting 

 in in- and outflow of water into the surrounding sea water. 



