STARFISHES, URCHINS AND SEA-CUCUMBERS 59 



centre of the under side is the mouth with its five white teeth pro- 

 vided with powerful muscles Avhich enable them to grind the vege- 

 table food upon which the sea urchin lives. Tlie mouth leads into 

 a coiled intestine which opens by a pore at the summit of the 

 body. Around this opening in the areas devoid of spines, are five 

 little pores which are the openings of the reproductive system. 

 In summer the eggs are cast out through these pores, and float 

 in the water where they rapidly develop into little translucent 

 larvfe with reddish spots, and provided with eight long processes 

 which are rendered straight and rigid by calcareous rods. After 

 swimming about, and devouring small creatures for several weeks 

 the sea urchin suddenly develops in the posterior end of the larva, 

 most of the body of which is absorbed and withers away in a few 

 hours, leaving, upon the bottom, a minute globular sea urchin not 

 larger than a pin's head. 



The Green Sea Urchin, fSirongylocentrotus di'ohadticnsis, Fig. 

 25 J, is found in the deep waters of Long Island Sound but north of 

 Cape Cod it occurs in shallow tide-pools, and on the Maine coast it 

 literally covers the rocks. It extends into the Arctic Ocean, and is 

 found on the north' Pacific coast. It may be at once recognized by 

 its resemblance to a greenish chestnut bur. The body is flatter 

 than a hemisphere, about two inches in diameter and densely cov- 

 ered with sharp pointed spines not more than three-eighths of an 

 inch long. In some individuals these spines are tipped with dull 

 violet. In addition to the spines, there are also a large number of 

 little pincers mounted upon rod-like bases. These are used for the 

 removal, or retention, of refuse material Avhicli may fall upon the 

 sea urcliin. The five double rows of long, slender, tube-feet allow 

 the animal to cling to the rocks over which it slowly glides in search 

 of the algfe, and small organisms i;pon which it feeds. It often 

 covers itself witli bits of seaweed and other fragments. 



The Stinging Urchin, (Diadema setosumj, is abundant along 

 the Florida coast and in the West Indies, where it is often seen in 

 clusters upon the sandy bottoms. These urchins are velvety black, 

 about four inches in diameter, and are covered with sharp i:)ointed 

 black spines, some of which are fully four inches long. These 

 spines wave rapidly about upon the approach of an enemy, and if 

 the urchin be seized, they penetrate the skin of the attacking per- 

 son and break off, inflicting a most painful sting. 



