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Schemes for distribution of water vessels in adoral, ambital, and petaloid regions of the ambulacra are described. In the adoral and ambital region three basic patterns are described. All three function as connections between the accessory tube foot ampullae and the radial water vessels.
Lobe-like expansions or side spurs of the radial water vessels connect to the accessory tube foot ampullae in genera of the suborder Laganina. Simple unbranching lateral water vessels connect to the accessory tube foot ampullae in genera of the suborder Clypeasterina. Complex branching lateral
water vessels were observed in genera of the suborder Scutellina. Accessory tube feet help collect food and are closely associated with the food grooves. Four schemes of lateral water vessel and accessory tube foot distribution within petaloid areas are described, two observed in petals composed
of primary plates only, and two observed in petals with primary and demiplates. The respiratory tube foot/ampulla system of Dendraster excentricus is compared with the suckered tube foot/ampulla system of the regular urchin Strongylocentrotus purpuratus. The ampullae of both
systems are flattened, and very thin walled. Fluid constantly circulates between tube foot and ampulla. Unflattened bulbous ampullae were observed on the accessory tube feet and buccal tube feet of clypeasteroids and the buccal tube feet of the regular urchin S. purpuratus. These
bulbous ampullae connect to the respective tube feet through single pores. The greatly expanded ambulacral columns and adoral interruption of interambulacral columns are considered closely related to the development of the accessory tube foot system in clypeasteroids. These expanded ambulacra
and the adjacent regions of the interambulacra which support accessory tube feet are considered phyllodes and homologous to the more recognizable but less expansive phyllodes of the cassiduloids. Some characters are common to the cassiduloids and clypeasteroids strongly suggesting a cassiduloid
ancestry for the clypeasteroids. Among these are a monobasal apical system, respiratory pores between petal plates, the shape of lantern pyramids, buccal tube feet on basicoronal plates, expansion of the ambulacra forming phyllodes, and single pore supported tube feet adoral to the petals.
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