506 



RADIATES : ECHINODERMS. 



Fig. 491. 



ab-oral region over the oral, 

 the former being developed 

 into a cup or calyx-like pro- 

 jection, and composed of sol- 

 id, immovable plates. With 

 one exception, the living rep- 

 resentatives bear a great 

 resemblance to Star-Fishes 

 and Ophiurans ; but the fos- 

 sil species, which are exceed- 

 ingly numerous, and literal- 

 ly fill the rocks in many re- 

 gions, and whose beauty and 

 ornamentation are beyond 

 description, have a long, 

 jointed stem, and are popu- 

 larly known as Stone Lilies. 

 These stemmed crinoids, so 

 abundant in the past, have but a single representative in 

 the present seas, the Pentacrinus caput-mcdnsce of the 

 West Indies. Comatula has a stem in its early stages, 

 thus for a time appearing like the old crinoids ; but at 

 length it becomes detached, and spends the remainder of 

 its life as a free crinoid. 



SECTION II. 



THE CLASS OF ACALEPHS, OR JELLY-FISHES. 



THE Class of Acalephs comprises jelly-like radiated 

 animals, with a central cavity hollowed out of the mass 

 of the body, --which is generally built of four, eight, or 

 twelve spheromeres, and this cavity with a central oral 

 opening ; and instead of radiating plates, as in the next 

 order, there are radiating tubes, which pass from the cen- 



