This is a heart urchin, most likely Lovenia cordifornis. This particular species is relatively shallow-water dwelling, from the intertidal down to 60 feet (18m). It has long guard spines that can be raised when disturbed and folded back when resting. It likes to bury under the sand, but it can scoot around on top of the sand, using its longer spines. They eat detritus, and can create a mucous strand to capture falling particles.
Unlike most urchins, the heart urchin’s test is very fragile, and can be broken easily. This is why they tend to live in deeper calmer waters and under sand and not in rocky areas where you find regular urchins. Like thier sand dollar relatives, they have a the five petaliods, through which most gas diffusion takes place. Unlike most other echinoderms, such as sea stars and 'regular' urchins, heart urchins are not radically symmetrical .