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Figure 11 | BMC Evolutionary Biology

Figure 11

From: Origin and evolutionary plasticity of the gastric caecum in sea urchins (Echinodermata: Echinoidea)

Figure 11

Homology of the sea urchin gastric caecum based on its integration into the mesenterial system, in particularl the dorso-ventral mesentery, as a primary criterion. Virtual horizontal sections based on MRI scans of Cidaridae (A), Aspidodiadematidae (B), Stomopneustidae (C), Temnopleuridae (D), Parasaleniidae (E), Parechinidae (F), Strongylocentrotidae (G), Echinoneidae (H), Cassidulidae (I), Schizasteridae (J), Loveniidae (K), and Spatangidae (L). The gastric caecum - where present - is attached to esophagus, axial complex, and the test through the dorso-ventral mesentery (arrows). In the Cidaroida (A), the dorso-ventral mesentery attaches to the single festoon present in ambulacrum III. In the more derived spatangoid and certain holasteroid taxa, this mesentery is shifted away from its original insertion near ambulacrum III towards interambulacrum 3, resulting in an oblique position of the gastric caecum (K, L). Note that the gastric caecum, in contrast to the rest of the digestive tract, is always free of sediment grains in the burrowing irregular taxa (H-L). Not to scale.

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