The Echinoid Directory

Arachnoides Leske, 1778, p. 471

Diagnostic Features	Test with thin, sharp margin. Base flat, upper surface convex. 4 gonopores. Peripheral buttressing in the form of concentric walls around outer part of test only. Petals very broad and widely open distally; slightly elevated above adjacent interambulacral zones. Pore pairs in petals conjugate. Petals composed of alternating primary and demiplate throughout. All interambulacra disjunct orally; separated by three pairs of ambulacral plates. First post-basicoronal ambulacral plates slightly larger than other plates on oral surface. Basicoronal circlet pentagonal with points formed by interambulacral plates. Only one pair of post-basicoronal interambulacral plates on oral surface. Periproct on aboral surface immediately adjacent to margin; opening bounded by second paired post-basicoronal interambulacral plates. Food grooves comprising simple perradial trunks only; extending aborally to the apex. Naked channel running from periproct to basicoronal plates. Pedicellariae trivalved. Ambulacral tubercles and pores arranged linearly, forming combed areas. Combed areas well developed on both oral and aboral surfaces.
Distribution	Plio-Pleistocene to Recent of the Indo-Pacific.
Name gender	masculine
Type	Echinus placenta Linnaeus, 1758, p. 666, by monotypy.
Species Included	A. placenta (Linnaeus, 1758); Plio-Pleistocene to Recent, Australia to Philippines. A. tenuis H. L. Clark, 1938; Recent, western Australia.
Classification and/or Status	Clypeasteroida; Clypeasterina; Arachnoididae; Arachnoidinae. Presumed monophyetic.
Remarks	Differs from the type species of Monostychia, Fellaster and Ammotrophus in having only a single pair of post-basicoronal interambulacral plates on the oral surface. Monostychia and Ammotrophus further differ in having their periproct oral in position. In addition, Fellaster has no naked furrow running from the periproct to basicoronal plates. Differs from fossulasterids in having broad and well-developed combed areas throughout. Mortensen, T. 1948. A Monograph of the Echinoidea IV.2 Clypeasteroida. C. A. Reitzel, Copenhagen. J. W. Durham 1955. Classification of clypeasteroid echinoids. University of California Publications in Geological Sciences 31(4), 73-198.