INVERTEBRATE ANIMALS. 53 



Cbtrtecntb ^ixhk €a^^ 



Tlie Elarh *^\ indicates specimens or utlipr illustrations exhibited in the Cases. 



Sub-Class ECHINOIDA. Sea-Urcliins. 

 sxhog, hedgehog ; i'Soc, form. 



Family CIDARIDJil. xldapig, a Persian turban. 



Group 145. — Genus CIDARIS (Lamarck) and allies. The 

 globose CidaridcB differ widely in appearance from the 

 long-armed Asteridce of the preceding order ; but if the 

 arms of a Star-fish were turned up over its back till the 

 tips nearly met, and if the sides of the turned-up arms 

 were united by plates bearing tubercles crowned with long 

 spines, the morphological affinity between the Star- fish 

 and the Sea-urchin might to some extent be thus illus- 

 trated. In the Sea-urchin five spaces with ambulacral 

 pores alternate with five spine-bearing segments, all 

 vertically disposed, the mouth being always on the under 

 side. Estimated number of species : recent, 17 ; fossil, 

 424. Chiefly from the Oolitic and Tertiary formations. 



If Cidaris ixipillata (Fleming), the only representa- 

 tive of the group in the British seas, was 

 formerly supposed to be found exclusively near 

 the Shetland Islands, where it is called the 

 Piper, its spines being compared with the 

 drones of a bag-pipe. 



Group 146. — Genus DIADEMA (Gray) and allies. S<«g»j|aa, 

 a diadem. The metamorphosis of an Echinus has 

 no parallel out of its own class. The egg of the Sea- 

 urchin produces a free-swimming ciliated embryo, which 



