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The external and internal morphologies of cidaroid and camarodont sea urchin primary spines are investigated giving an overview of the internal microstructure and structural properties. The investigated species comprise the cidaroids Eucidaris metularia, Phyllacanthus imperialis, Plococidaris verticillata and Prionocidaris baculosa as well as the camarodont Heterocentrotus mammillatus (Echinodermata: Class Echinoidea), and morphological descriptions are based on scanning electron microscopy and micro-computed tomography. Stereom types and densities are differentiated using pore and trabecular diameter measurements. Structural analysis was performed using three point bending tests resulting in the calculation of force, deflection and stress, strain relationships. All studied species possess primary spines with a medulla consisting of laminar stereom regardless of the age and position of the spine on the tests. Differences in stereom morphology occur in the radiating layer and the surface of the spines. Material densities and stereom types differ with respect to growth lines when present and the radiating layer. The primary spines also show large differences in their outer morphologies ranging from smooth, striated to tuberculate. H. mammillatus spines are shown to bear more stress resistance than those of the cidaroids. Differences in spine morphologies and reaction to stress are interpreted with respect to functional morphological response, to ambient environmental parameters and their strategies between and within evolutionary stages.