ECHINODERMATA. OPHIUROIDEA. 73 



Distribution of the Ophiuroidea. 



The majority of the Ophiuroids at the present day 

 occur in shallow water, more than half of the known 

 species being found above a depth of 30 fathoms, and 

 most of these not extending lower. Other forms occur 

 at greater depths, as many as 69 species being found 

 below 1000 fathoms. Ophiuroids are rare as fossils, the 

 earliest form occurs in the Bala Beds, and belongs to the 

 genus Protaster, this ranges on to the Silurian, where 

 Eudadia and other forms appear. In the Mesozoic we 

 get Ophioderma, Ophiolepis, and Geocoma. All the Eocene 

 genera are still living. 



CLASS. ECHINOIDEA. 



The Echinoids or sea-urchins have usually a globular, 

 heart-shaped, or discoidal body. The soft parts are en- 

 closed in a shell or test, which is formed in the mesoderm 

 and is covered by a thin ectoderm. This test consists of 

 numerous calcareous plates, which in the majority of cases, 

 are immoveably united. There is nothing corresponding 

 to the ambulacral groove of the star-fish, the water- 

 vascular system being internal, as a result the tube-feet in 

 order to reach the exterior, must pierce the plates of the 

 test. The mouth is always on the inferior surface and 

 often central, but when excentral it is placed anteriorly. 

 The anus is either at the summit of the test or posterior 

 to it, somewhere along a line drawn from the summit to 

 the mouth. In the test we may distinguish two parts, a 

 small patch of plates placed at the summit, known as the 

 apical disc, and the remainder of the test termed the 

 corona. The apical disc represents the whole of the 



