STARFISH, SEA-URCHINS, AND SEA-CUCUMBERS. 49 



barrel-shaped creature, which usually has ciliated bands or 

 zones around its body; the Holothurian grows up in it, in 

 nearly the same way as the starfish. 



On review, we see that the Echinoderms differ from the 

 polyps in the body being more distinctly star-like or radiated; 

 that the skin is usually filled with solid limestone plates, 



forming a shell ; that they all have a tube-like digestive 

 canal, wliich lies loose and free in the body-cavity ; that in 

 the sea-urchin we first meet with teeth for cutting up the 



food; that Ecliinoderms can see and smell, and that most 

 of them pass through a metamorphosis. They are very com- 

 plicated animals compared with a sponge, a polyp, or even 

 a jelly-fish, having blood-vessels and gills. 



FIG. 51. Sea-cucumber (Synapta). A, larva. B, Young farther advanced, with 

 the Synapta (/. (/) growing within. C, young become free (after M tiller). 

 D, adult Synapta. (Kingsley del.) 



CLASSES OF ECHINODERMATA. 



1. Body mounted on a stalk Crinoidea (stalked starfish). 



2. Body with five arms; free Asteroidea (starfish, etc.). 



3. Body spherical, with long spines.. ..Echinovlea (sea-urchins). 



4. Body elongated ; skin soft, hardened 



by minute plates Holothuroidea (sea-cucumbers). 



4 



