824 MAKUAL OF NATURAL HISTORY. 



simple filaments disposed in a circular form round 

 the buccal orifice ; organs of body arranged in a 

 radiate manner round the digestive cavity ; not 

 amorphous or bilateral ; organs of digestion a simple 

 sac or short alimentary canal. Animal mostly 

 aquatic, breathing by gills, usually free. 



ECHINODERMS. 



Echinoderms constitute an exceedingly natural 

 and well-defined group of Radiate Animals, of 

 which Sub-kingdom they form the first and most 

 highly organized class. They derive their name 

 from the spiny covering with which many of them 

 are invested, whence also the popular designation 

 of the typical order, " Sea-Urchins/' Others again, 

 from a fancied resemblance between their rayed 

 outline, and the popular idea of the true figure 

 of the celestial bodies, are generally known as 

 " Star -Fishes/' and " Sea-Stars/' They are all 

 aquatic, without exception marine, and include 

 among their number some of the loveliest in- 

 habitants of the deep, the more stellate forms, 

 especially, appearing to the imagination as re- 

 flected images of the starry heavens. 



Being very generally distributed, they will fre- 

 quently arrest the attention of the observant tra- 

 veller in the progress of his researches along the 

 sea-coast and the neighbouring waters. Buried in 

 sand he will occasionally find Brittle-Stars (Ophio- 

 lepisy Ophiopholis, Ophiothrix), and in rock-pools 



