ABSTRACT
The microstructure of echinoderms has long been of interest (e.g. Müller 1854, Stewart 1871, Stock et al. 2004, Xiao et al. 2007). The different “Bauplans” of several sea urchin spines were compared by Hesse (1900), who classified them accordingly, and in doing so suggested that more recently evolved sea urchin spines are more simply organized than those of ancient taxa. Based on echinoid stereom, Smith (1980) differentiated and defined stereom microstructures such as rectilinear, galleried, labyrinthic, fascicular, perforated or even imperforate. These stereom types differ in general structure, orientation as well as the size and density relationships of struts to pore spaces. Labyrinthic sterom, for example, indicates a disorganized arrangement of trabeculae while galleried stereom shows long, connected, parallel galleries resulting in regularly arranged pores of uniform size. Stereom morphology can be related to different functions including substrates for muscle tissue insertion, load bearing surfaces or filling material.
