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Figure 3 | EvoDevo

Figure 3

From: How Hox genes can shed light on the place of echinoderms among the deuterostomes

Figure 3

Diagrammatic cross-section of a young post-metamorphic, generalized echinoderm (only the left side is portrayed). This presentation synthesizes developmental, morphological, and paleontology data [95, 96]. Typically, echinoderms begin development as bilateral embryos or larvae. At metamorphosis, a new structure (the rudiment) develops on the left side of the larva. The elaboration of the rudiment involves three main coelomic cavities, which during metamorphosis turn to stack postero-anteriorly: right somatocoel (dark blue), left somatocoel (light blue), and hydrocoel (red). The hydrocoel takes on a ring shape and produces five radiating extensions (the primary lobes) that will give rise to the primary podia, which will in turn mark the extremity of the five growing radial water vessels. The adult water vascular system, derived from the hydrocoel, comprises the circumoral ring and the radial water vessels. The tube feet (secondary podia) are lateral buds branching on those radial water vessels. In the oral direction, the left somatocoel develops five extensions that proceed through the hydrocoelar ring, separate from the left somatocoel and fuse to form the hyponeural sinus (dental sacs in echinoids). From this sinus will develop the hyponeural radial canals. More anteriorly, the epidermal layer thickens and bends into five epineural folds, which will give rise to the nerves (green) and epineural sinuses (orange), following the same pattern as the hydrocoel: circumoral (epineural and nerve rings), and radial elements (epineural canals and radial nerves). In summary, there are two main regions to a developing echinoderm. One incorporates right and left somatocoels and develops from the non-rudiment region of the larva; it corresponds to the extraxial part of the body (sensu EAT [95, 113]). The other incorporates the hydrocoel (water vascular system), the hyponeural elements and the epithelial derivatives (nerves and epineural strands) and is derived from a rudiment that develops laterally on the larva; it corresponds to the axial part of the body (sensu EAT).

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