ECHINODERMS OF THE CRAG. 



Sub-Kingdom. — E ADI ATA. 



Class.— ECHINODERMATA. 



There are Eight Orders of Echinoderms, which, arranged in descending sequence, 

 will stand thus : — 



1. SlPUNCL'LIDEA. 5. OPHIURIDEA. 



2. HOLUTHUKIDEA. 



3. ECHINIDEA. 



4. ASTERIDEA. 



6. Blastoidea. 



7. Cystidea. 



8. Crinoidea. 



Of the Sipunculidea and Holothuridea no species are found in the fossil state, nor, 

 considering the organisation of the creatures composing them, are any likely to be so 

 preserved. The Blastoidea and Cystidea are exclusively fossil orders, and confined to 

 Palaeozoic strata. The Crinoidea are for the most part fossil, and chiefly Palaeozoic and 

 Mesozoic. The Echinidea, Asteridea, and Ophiuridea are, on the other hand, chiefly 

 Recent, Tertiary, and Mesozoic, and but poorly represented in the earlier strata. 



The Echinoderms, of which fossil remains occur in the Crag, are either Echinidea, 

 Asteridea, or Crinoidea .■ chiefly members of the first order. It is quite possible that, 

 many species of star fishes and of brittle stars {Ophiuridea) lived in company with them, 

 although, owing to their excessive fragility, no remains are preserved to us. 



Order.— ECHINIDEA. 



The Echinoderms of this order are distinguished by their regular plated test, investing 

 the entire body with a coat of mail. They have always two digestive orifices. Their 

 ambulacra are always embodied in the test, and divide it into segments. Their genital 

 orifices are placed in the centre of peculiar plates, which form an apical disk, usually in 



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