Blastoids (sometimes called sea buds for their shape) have a long stalk with a head at the top. They went extinct about 260 million years ago (during the Permian), but they used to be abundant on Earth. Their heyday was during the Mississipian (about 350 million years ago). They lived anchored to rocks by their stalks, feeding by filtering small food from seawater. At the top of their head is a mouth, surrounded by grooves with tiny hairs for trapping food. The structure of their stalks and heads is what left a record of their existence. The stalk is made of stacked, calcified disks. The head is made of rows of tightly interlocking, calcified plates that often remain together in the fossil record. A set of holes in the top of head are evidence of the blastoid's unusual respiratory system. Folded structures (hydrospires) were used to extract oxygen from seawater that passed through the holes.

Echinoderms stand out as the only organisms on Earth with five arms or other elements spaced evenly around a central point. This pentaradial symmetry (penta = “five;” radial = “around a central point") is obvious in some echinoderms, such as sea stars or brittle stars that have five arms. It’s more subtle on others, such as the five rows of tube feet on a sea cucumber or five grooves on a sand dollar. This unusual symmetry of adult echinoderms is not found in juveniles. In fact, echinoderm larvae have two-sided (bilateral) symmetry like humans, and must undergo a metamorphosis to become pentaradial adults. Body plans of animals during their development often say something about their ancestry. The bilateral bodies of developing echinoderms are a reminder that, aside from other back-boned animals (vertebrates), echinoderms may be our closest relatives. Unlike most living echinoderms, many fossil echinoderms did not have pentaradial symmetry.