| As an initial matter, the echinoderms and chordate phyla have been arranged in almost every possible phylogenetic order. |
| Molecular phylogenies have suggested that hemichordates are in fact a sister lineage to the echinoderms. |
| Others have maintained that the earliest brachiate echinoderms had only three arms. |
| Marine invertebrates include ammonites, echinoderms, bivalves and crustaceans, but infaunal elements are rare. |
| However, the placement of the ophiuroid branch with respect to the other echinoderms is still unresolved. |
| The calcitic ossicles of crinoids, as is typical of echinoderms, form an internal skeleton that provides support and protection. |
