Organs of the axial hemal complex in crinoids — structure and occurrence

摘要

Close to the oral-aboral axis of the crinoid body a complex of hemal spaces occurs. Depending on the local density of hemal spaces and to the presence of cells or epithelial tubules, three different organs have been described, named spongy organ, axial organ and Reichensperger's organ. This study gives an overview of the light- and electron-microscopy of these organs. In addition, species not yet investigated in regard of the presence of any of the above mentioned organs have been included in this study. Finally, a survey is given on the distribution of these organs in different crinoid taxa. Amongst the species studied so far, all comatulids have a spongy and an axial organ, as do all isocrinids, which in addition possess a Reichensperger's organ. The genus Holopus fails to have any of these three organs; Cyathidium has a special kind of small spongy organ. In the genera Bathycrinus, Democrinus, Caledonicrinus and Gymnocrinus, an axial organ could be detected, but no Reichensperger's organ. In conclusion, an axial organ with epithelial tubules included is present in all forms with a stalk or centrodorsale. Reichensperger's organ seems to be a peculiarity of isocrinids and the hyocrinid Calamocrinus, whereas a spongy organ – although differently elaborated – may be ubiquitous in crinoids. It is missing only in juveniles.