The slate-pencil urchin Helcrocenirotus mammiUatus, 

 with its great clubhke spines, is more than two liands- 

 ful in size. (Great Barrier Reef. Fritz Goro: Lije 

 Magazine ) 



parma reaches a diameter of approximately 3 inches 

 along the Atlantic coast from New Jersey northward, 

 as well as around the Pacific from Vancouver Island 

 to Japan. Its petaloids end abruptly, as though in- 

 complete. 



From Nantucket to Brazil, the sand dollar Mellita 

 testiidinata has pointed petaloids and develops 

 notches in the rim of the body. These become sur- 

 rounded as slots when the sand dollar grows. In 

 the Gulf of California, certain members of Encope 

 acquire even more distinctive slots and holes, and 

 are popular as "sea arrowheads." Echinodiscus auri- 

 tiis, the yellow or purple "sea pancake" of Africa's 

 east coast, excels most other sand dollars in size and 

 thinness of the body. The shells of many of these 

 animals can be ground up in water to make an indel- 

 ible ink. 



HEART URCHINS 



It is easy to tell a heart urchin from a sea biscuit, 

 even though both have a bulky oval body and a 

 heavy shell. The heart urchin has only four complete 

 petaloids on its aboral surface, and the mouth is a 

 transverse slot somewhat anterior of the middle on 

 the lower surface. The anus is well posterior. Inter- 

 nally the shell is braced, but the animal has no Aris- 

 totle's lantern. 



Heart urchins are burrowers, usually covered with 

 fine short spines that slant backward as though 

 combed. If large spines are present, they too are 



aimed in a way that offers little resistance as the ani- 

 mal works along through the muddy bottom. 



Spaiangiis piirpiireus. a violet-colored animal, ex- 

 cavates chambers for itself in the mud along shores 

 in western Europe, the Mediterranean and the west 

 side of Africa. Its presence can be suspected from the 

 small hole kept open between its chamber and the 

 water above. Mucus secreted by the heart urchin 

 keeps its cavity from shedding mineral particles while 

 extremely long tube-feet are extended from the petal- 

 oid areas through the hole in the chamber's roof to 

 scavenge for food over adjacent areas of bottom. 



The Serpent Stars 



( Class Ophiiiroidea ) 



By far the most active of all echinoderms are the 

 serpent stars. Yet because most of them are of small 

 size and retiring habits, they are less familiar than 

 sea stars and sea urchins. They are often called "brit- 

 tle stars" because of their readiness to throw off parts 

 of their arms when disturbed. Each arm may break 

 into many pieces. A few kinds go so far as to dis- 

 card the upper part of the body as well. Then the 

 missing portions are regenerated. 



The five arms of a serpent star (only a few have 

 six or seven arms ) are distinct from the disk-shaped 

 or pentagonal body. On the oral surface they lack the 

 grooves as well as the sucker-tipped tube-feet found 

 in sea stars. Only a pair of minute soft swellings at 

 each joint in the arms represent the tube-feet. Ap- 

 parently they are primarily sensory. 



Flexibility of the arms in most serpent stars is 

 mostly in a horizontal direction. The animal curls 

 them around irregularities of the bottom and lifts its 

 body along, ordinarily holding it above the surface 

 on which the arms rest. The arms commonly are five 

 to six times as long as the diameter of the body, but 

 in some serpent stars the proportion reaches as much 

 as fifteen times. 



The arms are so flexible that they suggest a ser- 

 pent's tail, giving the common name to the animals 

 and also the word for the class to which they belong 

 (from aphis, a serpent, and iiru, the tail ) . 



Serpent stars frequently cluster together in aston- 

 ishing aggregations. The tangle of arms becomes im- 

 pressive enough to suggest that the group forms a 

 moie efficient trap for suspended food matter than is 

 possible for a single individual. 



The mouth at the center of the oral surface has five 

 sharp teeth but no Aristotle's lantern. Close to the 

 mouth is the opening through which the water-vascu- 

 lar system is filled. 



Serpent stars feed on a wide variety of bottom ma- 

 terial and take advantage of opportunites to include 



284] 



