400 



ECHIN0Z0A. 



portance of the group from a palaeontological point of view is not 

 sufficient to justify its being treated of here except as regards its 

 general features. The upper surface of the disc of an Ophiuroid 

 is covered throughout with calcareous plates, scales, or granules. 

 A large central plate (" dorsocentral") is sometimes recognisable, 

 and a pair of large plates (" radial shields ") is usually developed on 

 the dorsal aspect of the disc at the point of origin of each of the 



Fig. 276. — Ophiuroidea. Ofihioglypha laccrtosa. a, Outline, of the natural size ; B, The disc 

 viewed from above, twice the natural size ; c, The disc viewed from below, showing the mouth 

 and genital fissures, twice the natural size. (Original.) 



five arms (fig. 276, b). On the under side of the disc the arms, 

 with their ventral rows of plates, are continued to the mouth, but 

 the spaces between the arms ("interbrachial areas") are covered 

 with plates, scales, or minute tubercles of lime (fig. 276, c). The 

 stellate aperture of the mouth is bordered with a system of peristo- 

 mial ossicles, some of which act as teeth ; while at its angles, one in 

 each interbrachial space, are situated five large " mouth-shields " 



