163 



CHAPTEE YIII. 



SEA-URCHINS AND STAR-FISHES. 



COMMON ECHINUS. — MEANS OF PROGRESSION. — PROPORTIONS OF PLATES. — 

 DIGESTIVE ORGANS. — ECHINUS MILIARIS. — SPATANGUS. — CIDARIS, — 

 STAR-FISHES. — THEIR CLASSIFICATION. — THE ROSY FEATHER-STAE. — 

 OPHIOCOMA. — LUIDA. — URASTEE. 



Sea-Uechins are distinguished from other Echinoderms by 

 their more or less rounded form^ and by their shelly box or 

 cqse^ which is composed of a number of plates united edge- 

 wise by integuments. On the shelly plates are tubercles of 

 various sizes^ rounded at their tops, forming halls corre- 

 sponding with sockets in the ends of various spines,, which 

 are movably attached to them by that kind of joint. The 

 spines differ in size, number, arrangement, and shape, ac- 

 cording to the genus or species to which the specimen be- 

 longs. Radiating down the sides, and dividing the box 

 like the quarterings of an orange, are a kind of canals or 



