North American index fossils, invertebrates . ierced by two large openings which inlife are covered, except at their centers, by leathery membranesstudded with small calcareous particles; these are the mouth open-ing or peristome, and the anal opening or periproct. The mouth is on the under or actinal side of the test, eithercentral or excentric in position and is surrounded by a leatherymembrane. It contains in life a very complicated dental apparatus{Aristotles lantern) consisting of five hard, interradially situ-ated teeth which are in relation with as many pyramids resting ECHINODERMA TA—E
North American index fossils, invertebrates . ierced by two large openings which inlife are covered, except at their centers, by leathery membranesstudded with small calcareous particles; these are the mouth open-ing or peristome, and the anal opening or periproct. The mouth is on the under or actinal side of the test, eithercentral or excentric in position and is surrounded by a leatherymembrane. It contains in life a very complicated dental apparatus{Aristotles lantern) consisting of five hard, interradially situ-ated teeth which are in relation with as many pyramids resting ECHINODERMA TA—ECHINOIDEA. 573 upon the membrane. Muscles connect the pyramids with eachother and with projections from the soHd test surrounding it. The anus is surrounded by a soft membrane similar to thataround the mouth and is placed either at the center of the apicalsystem or at a variable distance from it in the median line of theposterior interambulacrum, upon either the upper or the undersurface of the test. With mouth centrally placed and anus at. iamb Fig. 1911. Diagram of Linihia variabilis. A, dorsal view ; B, posterior view ;C, ventral view. (After A. W. Slocum.) «, apical system ; amb, ambulacra ; as,anterior sulcus ; iantb^ interambulacra ; If, lateral fasciole; /, petaloid part of ambula-crum ; />/, peripetalous fasciole ; //, periproct; ps, peristome. the center of the apical system, the test is said to be regular ortndocyclic; with mouth central or excentric, and anus excentric,the test is known as irregular or exocycUc. The plates of the test are arranged in ten meridian-like of these, the amhulacral areas, are composed of small per-forated plates, the remaining five interambulacral areas alternatewith these and are imperforate and usually larger. In the vastmajority of echinoids, each zone is composed of two columns ofplates, making twenty columns in all, ten perforate, and ten non-perforate; this number is, however, not attained in some fossilforms and is
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