Sea urchins are echinoderms, to which group belong also the starfishes and the beche-de-mer sea slugs. The most distinctive structure of an echinoderm is an elaborate water-pumping system which activates numerous feelerlike processes known as tube feet. Echinoderms have a network of calcareous plates embedded in tissue and muscle. In the sea urchins these plates are fused as a mosaic to form a rigid shell, the exterior of which is usually furnished with movable spines.
Typical of our local sea urchins are the common sea urchin or sea egg (Evechinus chloroticus), greenish to purplish, and densely spinose on a flattened circular “shell”, 4–6 in. across, which is light greenish when the spines are removed.
The heart urchin (Echinocardium australe), very thin shelled, 1–2 in. across, is found buried in mud from shallow water to about 16 fm.
The cake urchin or snapper biscuit (Arachnoides zelandiae), is a five-segmented shelly disc found in fine sand at entrances to harbours. The broken-up triangular segments are commonly cast up on beaches.
by Arthur William Baden Powell, Assistant Director, Auckland Institute and Museum.