Figure 1. Diagrammatic representation of the water vascular system of echinoderms (after Nichols, 1969).

Figure 2. Skeletal morphology of a typical, regular sea urchin (after Smith, 1984).

Figure 3. Simplified phylogenetic tree for the phylum Echinodermata (after Paul and Smith, 1984). The five living classes lie across the top of the diagram. Relationships of key early echinoderms are shown below. A–E, ambulacra A–E. Note that in Camptostroma, three ambulacral tracts radiate from the mouth, with the two lateral tracts dividing to give rise to ambulacra B + C and D + E. This is the primitive 2–1–2 arrangement of ambulacra thought to derive from the three ambulacra of the helicoplacoids.