58 



BRANCH ECHINODERMATA 



stars. The one most common on our shores (Ophiopholis) is 

 of a " general red hue spotted with brown and paler red." 



CLASS m. ECHINOIDEA 



The globular or disk-like sea-urchins have the pentameral 

 plan, as a cleaned " test " or shell (Fig. 44) will show. 



The body wall is composed of several hundred pentagonal 

 calcareous plates arranged in regular order in twenty rows, 

 the whole forming a sort of thin case or shell (see Fig. 44). 



Fig. 44. Sea-urchin (Echi'nus micros' toma) with spines nearly all removed 

 from " test." (Chapin and Rettger.) 



The ossicles, or plates, are armed with very long sharp spines 

 for defense (Fig. 45). Alternate rows of plates are perforated 

 for the passage of the tube-feet, there being no grooves. These 

 ten rows of perforated plates constitute the ambulacral areas, 

 and the ten rows of unperforated plates constitute the inter- 

 ambulacral areas. 



Color. The colors are brown, olive, purple, red, green, or blue. 



The protective resemblance is good. 



The ambulacral system of the sea-urchin is similar in plan 



