The plastron is the name given to the enlarged interambulacral zone on the lower surface behind the mouth. The arrangement of plates in this region provides critical taxonomic data. The first interambulacral plate and the one bordering the peristome is the labrum (L). The shape of the labrum is quite variable, ranging from crescentic to blade-like. The next two plates may be enlarged as sternals. In many spatangoids the two sternals are approximately equally developed and form virtually the entire plastron, as above. The plastron is then said to be amphisternous. However, sometimes the adjacent ambulacral plates may meet centrally so as to separate the labrum from the succeeding sternals: the plastron is then said to be disjunct. There may be a second paired set of plates behind the sternals, the episternals. Finally, a single interambulacral plate may be found towards the posterior of the plastron in some holasteroids and spatangoids; this is the rostral plate.
The plastron may be entirely covered in tubercles and spines, or tubercles and spines may be confined to the posterior part only.
A few of the more important plastron types are shown on the next page.