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 fewer than five 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 sixth 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）被用于研究步带骨板与口板的同源性，因其可预测轴骨骼中的骨板终端分支与终端增生现象，尽管该理论针对前者的例外情况可能出现在部分古生代棘皮动物（Paleozoic echinoderms）中。多样的形态设计与异常个体也可为骨板同源性验证提供依据。步带同源性已被广泛认可，其水孔（hydropore）及/或单个生殖孔（gonopore）位于卡彭特定义的CD间辐区。在2-1-2步带模式中，无分支的步带始终对应卡彭特A辐区。所有步带形态仅需三条核心指令即可实现：“生长”“分支”与“停止”。棘皮动物步带数量少于5个的变异范围表明，“分支”与“停止”两条指令在全部5个辐区内均独立发挥作用。步带数量可与口板数量存在相关性，也可无关联。“海林檎类（cystoid）”的口板存在两种基本模式：其一为每个步带对应一块围口（circum-oral, CO）板，外加CD间辐区的第六块骨板；其二为全部六块间辐口周（interradial peri-oral, PO）板，其中CD间步带区包含两块。拥有5块“口板”的类群可能丢失了PO3或PO6骨板。直立的步带结构在埋藏学（taphonomy）过程中最先被破坏，因此相关化石记录极为匮乏。所有步带骨骼结构与步带软组织均具有一致的拓扑学位置关系。当存在分支步带时，其主干或底板骨板通常会先特化为腕板（brachiolars）或羽肢板（pinnulars）。腕羽（brachioles）与羽枝（pinnules）均可从一块或两块底板骨板上发育出的关节面萌生。骨板终端增生、腕羽/羽枝的间距分布，以及盖板缺乏开合肌肉等特征，均表明“海林檎类”的步带内延伸有水管系统（water vascular system）。