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Echinidoes bilateralHeinke A G Schultz
De Gruyter, £200.00

When most people think of sea urchins, they picture a rounded, symmetric and circular animal with five-fold symmetry that lives in the sea. Although it is certainly correct that all sea urchins live in the sea, not all urchins have five-fold symmetry. In fact, not all urchins are rounded either. This is an academic reference book about the types of sea urchins that have a bilateral symmetry, with in some cases only the smallest hint of the five-fold symmetry of the familiar rounded sea urchins.

Sea urchins can live on the surface of the sea bottom from the high tide line down to several kilometres' depth. Those in the shallows tend to have thicker tests (hard shells) than those in deeper water.

Urchins all have spines, but many species lose these easily after their death, so look quite different if washed up on beaches. Live urchins can be almost any colour, from red, purple or black to brown or green, but most are white when their spines are lost.

Irregular urchins can be almost rounded, flattened or quite cushion-shaped, heart-shaped, bottle-shaped or very elongated. This tremendously detailed, well-illustrated and well-referenced book covers all these variations, and clearly describes identifying features of all known extant species, with information about the animals' behaviour and biology if known.

Dr Amanda Hardy MRSB