A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . s. Class BLASTOIDEA. Table-case The Blastoids (Bud-shaped) form a small class of Echino-derma, which arose at an early period, probably from Cystidea Diploporita, and flour-ished chiefly in Devo-nian and Carboni-ferous times. TheMuseum possesses arich collection, whichserved as basis of amonograph wiitten byEtheridge and Carpen-ter and published bythe Trustees. A listof the specimens hasalso been the fossils aretoo small to be seenclearly, only a fewc
A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . s. Class BLASTOIDEA. Table-case The Blastoids (Bud-shaped) form a small class of Echino-derma, which arose at an early period, probably from Cystidea Diploporita, and flour-ished chiefly in Devo-nian and Carboni-ferous times. TheMuseum possesses arich collection, whichserved as basis of amonograph wiitten byEtheridge and Carpen-ter and published bythe Trustees. A listof the specimens hasalso been the fossils aretoo small to be seenclearly, only a fewcharacteristic exam-])les are general appear-ance of the blastoidskeleton and the termsapplied to its moreobvious parts areshown in Fig. 30. Thebrachioles border fivefood-grooves, of whichthe skeleton is rathercomplicated. The con-tiguous edges of theplates termed deltoidsand radials werefolded, and in most ofthe genera these foldsprojected far into theinterior of the thecaand thus enabled the Fig. typical Blastoid, Orophocrimis fusi- ^g^ated sea-water to^r^^s, Carboniferous (Kmderhook) of Iowa, ^^^^^ ^^^^^. Stem Root ECHINODEEMA—BLASTOIDS. 67 internal organs; hence these folds are called hydrospires Gallery(water-breathers). In most blastoids the theca is borne on stem and shows conspicuous five-rayed symmetry. In Table-easea few forms, however, the theca rested on the sea-floor, andthis produced irregularity in its shape with a change in oneof the food-grooves; examples of this are Eleiitlierocrinus,Pentephylliom, and Zygocrinus, which in other respects aredissimilar and not closely related. The preceding Classes are essentially fixed forms, livingwith the mouth upwards and obtaining food by means of acurrent of sea-water swept towards the mouth along ciliatedfood-grooves. They are therefore termed PELMATOZOA(stalk animals) in opposition to the ELEUTHEROZOA (free-moving animals) such as star-fish and sea-urchins, which livewith the mouth downwards
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