petals of a tulip. The disk of the body becomes 

 rounded and the animal is no longer in balance. It 

 topples to one side, and then proceeds to curl the un- 

 der arms and take hold of the bottom with its tube- 

 feet. Eventually it reaches the ordinary outspread po- 

 sition, mouth downward. 



Other stars use the tulip method in reverse, bend- 

 ing the arms away from the mouth, rising up like an 

 inverted flower until they topple or can grasp the bot- 

 tom with extended tube-feet on some one or two 

 arms. 



A great many stars, when righting themselves, ac- 

 complish the same end with far less obvious move- 

 ment. Just the tips of one or two arms (usually two) 

 curl under, away from the mouth side of the animal. 

 Their tube-feet gain a hold on the bottom and, with 

 this beginning, the star proceeds to walk under itself, 

 the region of bend shifting nearer and nearer the disk 

 of the body. Finally the remaining arms may be raised 

 free of the bottom, and the slow somersault is com- 

 plete. Or the folding may continue all the way to the 

 tips of the opposite arms, none of them ever being el- 

 evated into the water. 



The tube-feet of sea stars are stout organs. Yet, un- 

 less the star is climbing a vertical surface, they appear 

 to push rather than to pull. Muscles in their walls 

 serve to aim the tube-foot as it is extended by hydrau- 

 lic pressure from the water-vascular system. Contrac- 

 tion of longitudinal muscles shortens the tube-foot 

 and expels the water. 



It is on tube-feet slanting away from the animal in 

 a backward direction that real force is applied. As 

 these tube-feet are inflated with water, they push the 

 body along very much as a man's feet push against the 

 ground in walking. A sea star strides very slowly on a 

 multitude of tube-feet all out of step with one another. 

 But in this way it can walk on soft mud as well as on 

 hard surfaces to which it clings. 



At the end of each arm, a star has one or more 

 tube-feet of a different sort. They lack suckers and ap- 

 pear to be feelers, especially sensitive to vibrations 

 and chemical substances in the water. With them a 

 sea star can be repelled by a salt crystal or attracted 

 to a piece of clam. 



In all but certain deep-sea starfishes, each arm has 

 at its tip a small cushion-like area that bears a cluster 

 of simple eyes. In most species this light-sensitive area 

 appears as a red spot. Often a creeping star curls the 

 tips of its arms upward, as though to peer vaguely in 

 the direction of movement by aiming the eyespot at 

 the surrounding bottom. 



Nearly 2000 ditt'erent species of sea stars have been 

 discovered, the greatest number from northern parts 

 of the North Pacific Ocean. Living sea stars all belong 

 to three great orders, separated upon inconspicuous 

 details of the pedicellariae and skeleton. Familiar and 

 remarkable sea stars are included in each of the three. 



THE EDGED SEA STARS 



A majority of deep-sea stars belong to the order of 

 "edged sea stars" ( Phanerozonia), the record for 

 depth being held by Albatrossaster richardi, dredged 

 from 19,700 feet below the surface near the Cape 

 Verde Islands. This order has many members too in 

 waters a beachcomber or skin diver can reach. 



Edged sea stars usually have a sharp boundary be- 

 tween the upper and lower surfaces. Along the mar- 

 gin of the often broadly joined disk and the arms, es- 

 pecially large skeletal plates commonly form two rows 

 (Ceramaster. Plate 130). These marginal plates, to- 

 gether with the ones that cover the upper surface with 

 a kind of mosaic pavement, give rigidity to the sea star. 



Many edged sea stars have pointed tube-feet with 

 no suction tip, and live normal lives with neither ped- 

 icellariae nor an anus. These are all features of the 

 common, sluggish mud star Ctenodiscus crispatiis, 

 and of the various kinds of Psilaster, Astropecten, 

 and Leptychaster encountered along muddy coasts of 

 the northern hemisphere. 



Ctenodiscus crispatiis itself is a short-armed, blunt- 

 tipped creature with a broad yellow disk. It sinks it- 

 self just below the surface of mud flats from shore to 

 depths of at least 6000 feet along coasts of both the 

 North Pacific and North Atlantic. Full size — 3 to 4 

 inches across — is probably reached by the time it is 

 three years old, showing that the mud star is really 

 efficient at extracting food from the sediments car- 

 ried to its mouth by a veil of mucus propelled by 

 the ciliated cells of the skin. 



The arms of Psilaster, Astropecten, and Liiidia 

 (Plate 123) are pointed and far longer than those of 

 Ctenodiscus. Psilaster andromeda needs about four 

 years to reach the full 4-inch spread of its slender 

 arms, feeding on small urchins, little clams, mussels, 

 and microscopic life in surface sediments. Recently- 

 dead Psilaster are often washed ashore, for they live 

 in waters as shallow as 60 feet and from there to more 

 than 2500 feet below the surface on both sides of the 

 North Atlantic — from Delaware Bay to Greenland 

 and down the eastern shores to the Cape Verde Is- 

 lands off the westward bulge of Africa. A gelatinous 

 secretion on the aboral surface of this star makes it 

 slimy to the touch. 



The -pecten of Astropecten is the comblike fringe 

 of spines attached to the marginal plates. Many of 

 these stars are large ones. A. articulatiis and A. cin- 

 gulatus both reach 10 inches in span. The former 

 lives in comparatively shallow water from New Jersey 

 to the Gulf of Mexico, and can be bright orange or 

 purple above and yellow below with orange-red mar- 

 ginal plates and purple spines. A. cingulatus inhabits 

 deeper water and usually is colored more drably. 



Still other species of Astropecten are the common- 

 est shallow-water sea stars in the Mediterranean. 

 They compensate for lack of suction cups on the 



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