. The Cambridge natural history. Zoology. SPATANGOIDEA 557 vertical bands of plates composing the corona is variable, in a word, that the corona has not yet acquired a fixed definite con- stitution. One genus {Echinocystites) has the anus outside the apical system. It has four rows of pore-plates in each radius, and numerous rows of plates each with a single spine in the interradii. Another {Palaeodiscus) has been shown by Sollas^ to be in many respects the missing link between Asteroidea and Echinoidea. Inside the plates of the corona there is a series of ambulacral plates like those of Aster
. The Cambridge natural history. Zoology. SPATANGOIDEA 557 vertical bands of plates composing the corona is variable, in a word, that the corona has not yet acquired a fixed definite con- stitution. One genus {Echinocystites) has the anus outside the apical system. It has four rows of pore-plates in each radius, and numerous rows of plates each with a single spine in the interradii. Another {Palaeodiscus) has been shown by Sollas^ to be in many respects the missing link between Asteroidea and Echinoidea. Inside the plates of the corona there is a series of ambulacral plates like those of Asteroidea. The tube-feet in the oral portion of the radii seem to have issued between the (outer) ambulacral plates. 'So anus has been detected. All the rest are Endocyclic. The oldest known form, Bothriocidans (Fig. 252, A), from the Ordovician, has only one row of interambulacral plates and two of ambulacral; no peri- stome is distinguishable from the corona. The Archaeocidaridae appear in the Devonian. They have narrow ambulacra of two rows of pore-plates as in the Cidaridae, but the interambu- lacra consist of many rows, the members of which overlap, and therefore were probably slightly movable, as in the Echinothu- riidae; the primary tubercles are large, and there is only one on each plate. The Melonitidae (Fig. 252, B) appear in the Carboniferous. Each interambulacral plate, of which there may be five rows in each interradius, bears numerous small tubercles, and there may be four or more vertical rows of pore-plates, though in the genus figured, Falaeoechinus, there are only two. The Tiarechioidae are represented by one genus, Tiarechinus, with an enormous apical system, from the Triassic of the Tyrol. The. ,jn)l> -inter Fig. 252.—A, Bothriocidaris. x ]. B, PalaeoecMnus, x 1. ainl}, Ambulacral plates ; inter, interambulacral plates. (After Zittel.) iterambulacra consist of one plate bordering the mouth, three, 1 "On Silurian Echinoidea and Ophiuroidea," Quart.
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
