Data from: Testing for homologies in the axial skeleton of primitive echinoderms

The Extraxial Axial Theory is used to investigate homology of ambulacral and oral plating because it predicts terminal branching and terminal addition of plates in the axial skeleton, although exceptions to the former may occur in some Paleozoic echinoderms. The variety of morphological designs and anomalous individuals also provide tests of plate homology. Homology of ambulacra is generally accepted, with the hydropore and/or single gonopore in Carpenter’s CD interray. In the 2-1-2 ambulacral pattern the unbranched ambulacrum is always in Carpenter’s A ray. All ambulacral morphology requires just three instructions: ‘grow’, ‘branch’ and ‘stop’. The range of variation in echinoderms with <5 ambulacra implies that both the ‘branch’ and ‘stop’ instructions acted independently in all five rays. Numbers of ambulacra may or may not correlate with numbers of orals. Two basic patterns of ‘cystoid’ oral plating occur; with a single radial (circum-oral, CO) plate from each ambulacrum plus a 6th in the CD interray, and with all six interradial peri-oral (PO) plates, with two in the CD interambulacrum. Five ‘orals’ may involve loss of PO3 or PO6. Erect ambulacral structures are lost first in taphonomy and so poorly known. All ambulacral skeletal elements bear the same topological relationship to ambulacral soft tissues. Where branched ambulacra occur, the trunk or flooring plates are often modified first brachiolars or pinnulars. Both brachioles and pinnules may arise from facets developed on one or two flooring plates. Terminal addition of plates, spacing of brachioles/pinnules and lack of musculature to open cover plates all suggest that ‘cystoids’ had extensions of the water vascular system in their ambulacra.

轴外轴学说（Extraxial Axial Theory）被用于探究步带与口板的同源性，因其可预测轴骨骼的末端分支与末端新增骨板，尽管前者的例外情况可能见于部分古生代棘皮动物。多样的形态设计与异常个体同样可用于检验骨板同源性。步带的同源性已被广泛认可，其水孔（hydropore）和/或单个生殖孔（gonopore）位于卡彭特CD间步带区（Carpenter’s CD interray）。在2-1-2步带模式中，不分叉的步带始终位于卡彭特A射线区域。所有步带形态仅需三条指令即可实现：「生长」、「分支」与「停止」。步带数量少于5的棘皮动物其变异范围表明，「分支」与「停止」两条指令在全部五条射线中独立发挥作用。步带数量或许与口板数量相关，亦可能无关。「海林檎类（cystoid）」口板存在两种基本模式：其一为每个步带对应一块辐（围口，CO）板，外加CD间步带区的第六块板；其二为全部六块间辐围口（PO）板，其中CD间步带区包含两块。五块「口板」的情况可能源于PO3或PO6的丢失。直立步带结构在埋藏学（taphonomy）过程中最先遭到破坏，因此相关记录极为匮乏。所有步带骨骼结构与步带软组织均具备一致的拓扑关系。当存在分叉步带时，躯干或底板往往会率先特化为腕板（brachiolars）或羽枝板（pinnulars）。腕与羽枝均可由一块或两块底板上发育的关节面衍生而来。骨板的末端新增、腕板/羽枝板的间距，以及盖板缺乏开启所需的肌肉组织，所有这些特征均表明「海林檎类」的步带中存在水管系统（water vascular system）的延伸结构。