ECHINODERMA. OPHIUROIDEA 115 



Distribution of the Ophiuroidea 



At the present day the majority of the Ophiuroids 

 live in shallow water, more than half of the known species 

 being found at a depth of less than 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 known 

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

 Protaster; this genus ranges on to the Silurian, where 

 Laptvorthura, Eucladia and others appear. In the De- 

 vonian, Furcaster and several other genera are found ; in 

 the Trias, Aspidura. In the Jurassic Geocoma, and forms 

 which have been referred to the existing genera Ophiura, 

 Ophiolepis and Ophiocten are present. In the Cretaceous 

 Ophiura and Amphiura are found. A few, such as 

 Ophioglypha, occur in the Eocene. 



CLASS III. ECHINOIDEA 



The echinoids or sea-urchins have usually a globular, 

 heart-shaped, or discoidal body, covered with spines. The 

 shell or test is covered by a layer of ectoderm and consists 

 of numerous calcareous plates, which, in the majority of 

 cases, are immovably united. Nothing corresponding to the 

 ambulacral groove of the star-fish is to be seen on the surface, 

 since the water-vascular system is internal to the skeleton, 

 and as a result the tube-feet, in order to reach the exterior, 

 must pierce the plates of the test. With one exception the 

 mouth is always on the inferior surface ; it is either central 

 or placed in front of the centre. The anus is either at 

 the summit of the test or posterior to it, somewhere along 



8—2 



