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Synoptic Keys to the Genera of Ophiuroidea

Class Asterozoa — Subclass Ophiuroidea

Class Asterozoa
Subclass Ophiuroidea

Asterozoa in which the alimentary organs are essentially confined to a central disc from which the arms are distinctly demarcated and capable of performing the locomotor movements, the tube-feet serving as non-suctorial, sensory tentacles.

In all existing genera the ambulacral plates fuse in pairs early in life to form median articulating joints, termed vertebrae, and the ambulacral groove is converted into an internal epineural canal. These characters sharply distinguish extant ophiuroids from other Asterozoa, but they are lacking from the more generalized Palaeozoic ophiuroids.

Key to the Orders of Ophiuroidea

1 (4)

An open ambulacral groove traverses the lower surface of the arm.

2 (3)

Ambulacral plates persist as discrete elements in the adult stage, not fusing together in pairs to form vertebrae; the arm is therefore capable of only limited movements. . . Order STENURIDA Spencer.

(An exclusively Palaeozoic group of 9 genera arranged in 5 families. See Spencer (1951)).

3 (2)

Ambulacral plates fuse in pairs to form vertebrae which permit snake-like arm-movements. . . Order OEGOPHIURIDA Matsumoto.

(Exclusively Palaeozoic, 7 families; see Spencer (1951)).

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4 (1)

Ambulacral groove closed over and purely internal, forming the epineural canal; ambulacral plates fused in pairs to form articulating vertebrae which permit snake-like arm-movements. . . Order OPHIURIDA Mueller & Troschel.

(The largest order, comprising all living species and all known fossils from the Triassic onwards, as well as one Palaeozoic family, the Aganasteridae.)