Yesterday I took a few close-up photos of a Purple Sea Urchin test (or skeleton).
You can see the rounded and raised tubercles to which the urchin spines are attached:
Also noticeable are the podial pores, or the holes from which the tube feet emerge.
Note that the pores are paired (two are placed side-by-side). One tube foot emerges from each pair of pores:
To provide some context for these anatomical features of the skeleton, here are two images of live urchins (an adult and a juvenile), highlighting the urchin spines and the extensible tube feet: