A MONOGRAPH 



ON THE 



FOSSIL ECHINODERMATA 



OF THE 



OOLITIC FORMATIONS. 



THE OPHIUROIDEA. 



The Ophiuroidea were long united with the true Star-fishes, Asteroidea, from the 

 circumstance that in both groups the arms proceed from the circumference of the disk ; 

 they are now, however, separated into a distinct order, as they possess certain permanent 

 organic characters by which they are distinguished from them. 



The Ophiuroidea have a central discoidal body, which is either naked or covered with 

 granules, spines, or scales ; in this is contained all the viscera, and from the mouth proceed 

 five long very flexible, simple, or ramified arms, sustained by a series of internal vertebra- 

 like pieces, covered by a naked integument or having granules, scales, or spines, developed 

 from the lateral or inferior parts thereof. The arms of the Ophiuroidea are widely different 

 from the rays of true Star-fishes, which are simple prolongations of the body of the animal, 

 whereas the arms of the Ophiuroidea are superadded to the body, and there is no excava- 

 tion in them for any prolongation of the digestive organs. There are no ambulacra] 

 grooves in the floor of the arms, nor any retractile tubular feet, nor pedicellarise, in this 

 order. 



The mouth, situated in the centre of the under surface of the disk, opens directly into 

 the stomach ; this is a sac with one aperture, the mucous membrane of which is covered 

 with vibratile cilia. 



The ovaries are placed near the arms, and open by orifices situated at the basal 

 surface of the interbrachial spaces near the mouth. 



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