Class ECHINOIDEA

<i>Echinometra mathaei</i> (Blainville, 1825)

Echinometra mathaei (Blainville, 1825)

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Class ECHINOIDEA

Sand Dollars, Sea Urchins

Compiler and date details

May 2012 - Tim O'Hara, Museum Victoria.

F.W.E. Rowe & J. Gates, Australian Museum, Sydney, New South Wales, Australia (1995) Updated (2001) by Tim O'Hara, Museum Victoria, Carlton, Victoria, Australia

Introduction

The Echinoidea, members of which are commonly known as sea urchins, heart urchins and sand dollars, comprise about 47 families, 189 genera and 800 species worldwide. Thirty-nine families, 99 genera and 214 species are known in Australian waters.

Echinoids are globular to disc-shaped, without arms but with moveable spines used in locomotion, and with a rigid case or test formed by 20 rows of abutting calcareous plates, five interambulacral areas of two plate rows alternating with five ambulacral areas of two plate rows, these latter being pierced by pores for the passage of the tube feet. Echinoids range up to about 165 mm diameter. Their spines occur in a variety of shapes and colours; some are multicoloured.

Echinoids are epibenthic on all substrates and some are infaunal in soft substrates. They are generally mobile grazers or burrowing detritus feeders. They occur at most depths, at least to 5000 m and are distributed worldwide. The group ranges from the Ordovician to Recent. The class Echinoidea is diagnosed as: free-living, radially symmetrical (globular) or secondarily bilaterally symmetrical (ovate to flattened, coin-like) echinoderms without arms.

The mouth is at or near the middle of the under (oral) side of the body but in some groups (e.g. spatangoids) the mouth may be closer to the anterior margin. In many groups the pharynx is ringed by a complex tooth, plate and muscle system known as 'Aristotle's Lantern' but in some major groups (e.g. spatangoids, cassiduloids, holasteroids) the 'Aristotle's Lantern' is absent. The anus is mid-upper (aboral) in position in regular echinoids and surrounded by a ring of genital plates, each pierced by a genital pore, and a ring of ambulacral terminal (ocular) plates. In secondarily bilateral forms the anus is separated from the genital and ocular plates and positioned on the posterior margin or sub-terminally on the oral side.

The water vascular system comprises a circum-oral ring canal and five radial ambulacral canals from which the ampullae/tube feet are given off laterally. The tube feet protrude through pores in the ambulacral plates, and the ampullae are internal to the plates.

A large coelomic cavity contains the coiled digestive tract and two to five gonads.

Endoskeletal plates abut, forming a firm test in most species; in a few deep-water species the plates overlap. The plates are surmounted by mobile spines and specialised stalked pedicellariae.

Echinoidea are dioecious, the sexes usually being indistinguishable externally, except for brooding forms. Reproduction is usually by broadcast of gametes; some species brood juveniles amongst their spines or in pouches in the test. Sexual dimorphism is known in brooding species. The feeding larva is an echinopluteus.

Detailed descriptive and illustrative information on the class Echinoidea can be obtained from a range of texts, including those listed in the 'References' below.

The classification adopted here is a modification of that given by Moore (1966), following Smith (1984).