The starfish Asterias forhesi extrudes its stomach to 

 envelop the small dead fish which will be its dinner. 

 (Virginia. Robert Bailey) 



out of sight when Pisaster comes within two feet of 

 them; half an hour after this star has passed by, they 

 come up again and resume feeding. They may have 

 an opportunity to repeat this vanishing act many 

 times, for Pisaster is believed to be one of the long- 

 est-lived of sea stars, reaching an age of twenty years. 

 In the Indo-Pacific, Coscinasterias calainaria is a 

 spinier star with from seven to fifteen arms. It opens 

 brachiopods as food, using the same method as 

 Asterias employs on clams. The giant twenty-rayed 

 Pycnopodia lielianllioicles of the Pacific coast from 

 southern California to the Aleutians often eats whole 

 sea urchins. In Puget Sound this sea star attains a di- 

 ameter of 22 inches. Apparently its mouth and appe- 

 tite are in proportion (Plate 128). 



The Ecliiiioids 



( Class Echinaidea ) 



Among the denizens of tide pools and seashores, 

 the sea urchins and sand dollars are second only to 

 sea stars as popular trophies. The smallest of them 

 are sand dollars barely V2 of an inch in diameter when 

 mature. The largest a skin diver can retrieve are black 



sea urchins with a shell 6 inches across; long, slender 

 spines usually add another 10 or 12 inches to the 

 space needed to accommodate the prize without 

 touching. In much deeper waters live urchins with 

 leathery, flexible shells almost a foot across. The 

 largest urchin known is a leathery specimen of 

 Sperostoma giganteum, taken off Japan; it measured 

 nearly 1 3 inches in diameter. 



Echinoids come in many difi^erent shapes: sea ur- 

 chins as regularly symmetrical as a doorknob; flat- 

 tened sand dollars and sea pancakes; broadly pointed 

 sea arrowheads; and heavy-bodied heart urchins. 

 Even their empty shells are things of beauty, for 

 without the skin covering them, they reveal a hand- 

 somely regular pattern of limy plates joined immov- 

 ably. Knobs on certain plates are the balls of ball- 

 and-socket joints for spines under individual muscu- 

 lar control — the armament from which echinoids 

 get their name, suggesting ecliinos, the hedgehog. 



As a sea urchin moves along a submerged rock or 

 the bottom, its spines seem constantly to be readjust- 

 ing themselves. Between them and often beyond them, 

 slender tube-feet may extend as feelers ready to detect 

 the approach of food or enemy. Other tube-feet bear 

 the weight, except in a few kinds of urchins in tropi- 

 cal waters that progress on their spines or lurch for- 

 ward by lifting themselves on their mouthparts. Their 

 tube-feet arise in five radiating areas, regions evident 

 in an empty, cleaned shell from the rows of circular 

 holes through which the tube-feet extended in life. 



Pedicellariae on long stalks also reach about, 

 seizing on particles and transferring them as though 

 hand to hand around the body to the mouth, or de- 

 fending the urchin against attackers. The commonest 

 type of pedicellaria among echinoids has three jaws 

 coming together only at the tip. Special ones are 

 glandular, with a poison sac in each jaw producing 

 a toxic material. Each pedicellaria has its own nerv- 

 ous control and can act independently in policing the 

 body surface. 



Echinoids replace tube-feet, pedicellariae, and 

 spines when these are damaged or lost. Cracks in the 

 shell can be mended, but new plates are produced 

 only during normal growth, keeping up with the en- 

 largement of the enclosed parts of the body. 



Probably sea urchins occasionally live to be older 

 than eight years, but their growth is most rapid while 

 young. The common green urchin Strongylocentrotiis 

 of the North Atlantic and North Pacific attains a 

 body diameter of about '4 of an inch by the end of 

 its first year, ■''s of an inch in the second year, 1 inch 

 at three years, Ws at four years, 2 inches at five 

 years, IVs at six years. Off the Norway coast this ur- 

 chin reaches a top diameter of about 3 inches. 



In sea urchins and sand dollars the mouth is 

 equipped with an amazing dredgelike device with as 

 many as forty separate pieces, serving to control five 



280 



