93 



wall these podia are represented by their ampullae and remark- 

 ably enough, all these little ampullae can be seen on dissection 

 to contain the same colored material. On the other hand this 

 system communicates freely with the coelomic fluid. The stone- 

 canal stiffened by carbonate of lime, which in other Echino- 

 derms and in larval Holothurians communicates freely with 

 the sea water, forms here the so called , .internal madreporites", 

 of which there may be one or even five, as in Holothuria 

 tubulosa. Furthermore the water vascular system stands in 

 connection with the tentacles, the tree-like feelers, which by 

 their active movements may assist greatly in respiration. 



The Polian vesicles are generally supposed to be the regu- 

 lators of the internal pressure of the water- vascular system. 

 Their wall contains muscle fibres and by a slight contraction 

 of these fibres the tentacles may be extended very easily. 



It seems however that it also plays a role in respiration. 

 The red material namely appears to be hemoglobin. The color 

 is due to the contents ; if one cuts a little hole in the wall of 

 the vesicle, the color ,,flows away". It can be centrifuged off 

 and a microscopical examination of the fluid reveals that the 

 color is bound to corpuscles. 



These corpuscles look under the microscope very much like 

 mammalian blood-corpuscles. They do not exhibit any active 

 movements as far as could be observed, have the same yello- 

 wish color that is characteristic for the erythrocytes and appear 

 to be perfectly spherical. A nucleus is in most cases clearly 

 visible, the protoplasm has a granular structure and a cell mem- 

 brane gives them a perfectly sharp outline. 



Hemoglobin is known to occur in many invertebrates. Though 

 it was originally thought to be characteristic for the verte- 

 brates, it has since been found in almost all groups of the 

 animal kingdom. Even in animals as low in the scale of phylo- 

 genesis as the worms, we find it in many forms, either in the 

 coelomic liquid (common earthworm) or in corpuscles (Glycera, 

 Capitella, Phoronis, etc.). In molluscs and arthropods it is gene- 

 rally found in solution. It is chiefly found in mud-dwellers or, 

 to put it more generaly, in animals, living in a medium where 

 oxygen is scarce. The larva of Chironomus is a classical example. 



Our knowledge of the occurrence of respiratory pigments 

 in Echinoderms is very incomplete. In one case, in the brittle 

 star, Ophiactis virens, hemoglobin has been found by F o e t- 

 tinger 39). Here it is also present in the tubes of the water 

 vascular system and in the Polian vesicles, two of these being 

 present in each interradius here. It is bound to spheroidal 

 corpuscles, which are sometimes flattened and eventually may have 

 the form of discs. By means of different chemical reactions, Fo e t- 

 t inger demonstrates that we have to do here with hemoglobin 

 The same pigment perhaps is also found in a species of Ophiolepis. 



