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BRANCH ECH I NOD ERM ATA 



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. — 8ea-urchiii {Echi'iiun micros' ioiiui) 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 amlnilacral areas, 

 and the ten rows of unperforated plates constitute the inter- 

 ambulacral areas. 



Color. — The colors are ])r()wn, olive, pur])le, red, green, or l)lue. 



The protective resemblance is good. 



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



