58 



BRANCH EOHINODERMATA 



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

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



CLASS III. 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' tunid) 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 plal(>s are perforated 

 for the passage of the tube-feet, tlu^re being no groovi^s. These 

 ten rows of perforated plates constitute the ambulacral areas, 

 and the ten rows of unperforated plates constitute the inter- 

 aml)ulacral areas. 



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



The protective resemblance is good. 



The ambulacral system of the sea-un^hin is similar in plan 



