stars, and sea cucumbers) or the outside world of 

 sea water. In the latter case it enters through a pore 

 connected to the ring canal by a slender tube (the 

 "stone canal") whose walls are stiffened by deposits 

 of lime. 



In sea cucumbers, sea urchins, and sea stars the 

 water-vascular system serves in locomotion. Its radial 

 canals communicate with an extensive series of short, 

 paired tubes, each with a muscular bulb and an elon- 

 gated, hollow tube-foot that projects from the body 

 surface. A tube-foot combines muscular and hydrau- 

 lic mechanisms. It is extended by contraction of the 

 bulb, forcing liquid into the cavity of the tube-foot; 

 muscles in the walls of the tube-foot control the di- 

 rection of extension. 



The tip of a tube-foot is a small, glandular suction 

 disk by means of which the echinoderm can attach 

 the sticky tube-foot to solid objects. Contraction of 

 longitudinal muscles of the tube-foot shortens it and 

 forces water back into the bulb, pulling the echino- 

 derm along or shifting the movable object to which 

 it holds. Teams of tube-feet also cooperate in carry- 

 ing the body, in policing its surface, or in supporting 

 bits of seaweed, rock, or coral as a shade against 

 strong sun in shallow water. 



Echinoderms maintain an important fluid in the 

 body cavity, taking the place of blood. It is shifted 

 from place to place by patches of cilia. In this fluid, 

 ameba-like cells move around freely, serving much 

 as white blood cells do in vertebrate animals. Some sea 

 cucumbers have red blood cells too, but with a hemo- 

 globin differing chemically from that of vertebrates. 



In their response to the environment, echinoderms 

 manage with a minimum of complexity in the nervous 

 system. It consists primarily of a ring around the 

 gullet, parallel to the ring canal of the water-vascular 

 system. From the nerve ring extend radial nerves 

 that branch profusely. In most echinoderms they end 

 blindly, yet appear able not only to coordinate the 

 movements of the animal but also to report on chemi- 

 cal substances in the surrounding sea, on conditions 

 of light and shade, on vibrations of many kinds. Free 

 nerve endings thus take the place of specialized sense 

 organs. 



Echinoderms generally are very casual about re- 

 production. In most instances, parents never meet. 

 They merely cast their eggs and sperms into the sea, 

 each to find the other by sorting themselves out in 

 the surface water from among a thin soup of repro- 

 ductive products of many species. Retention of the 

 eggs and some degree of maternal care are found in 

 each class, however; often they are correlated with 

 life in polar waters. 



Members of this phylum use many of the same 

 basic body features found in the vertebrates, but 

 emphasize each in a very different way. As in the 

 chordates and no other phylum of animals, they de- 



velop an internal skeleton. But instead of coordinating 

 that skeleton with muscles as a device important in 

 locomotion, they hide inside it as a shelter. Radial 

 symmetry seems to imply a readiness to withdraw, 

 moving away from molestation in any direction. Yet 

 with this different approach to life, often paralleling 

 ways found among coelenterates, the echinoderms oc- 

 cupy every habitat available in the seas, from oozy 

 muds at the greatest depths to the sandy beach and 

 the most wave-pounded rocky coasts. 



The Criiioids 



(Class Crinoidea) 



These delicate and often colorful creatures are 

 unique among echinoderms in that they live with the 

 oral surface uppermost. For food they depend upon 

 capturing small animals and plants drifting past them 

 in the sea, reaching out for this nourishment with 

 arms that may be more conspicuous than the body 

 to which they are attached. Usually each arm is 

 fringed along both sides with a row of short, tapering 

 branches, and suggests the fronds of a fern or the 

 petals of a lily. 



Most crinoids have five arms, each forked near the 

 base — producing ten flexible appendages. In many 

 species the arms continue to branch and rebranch 

 with increasing size of the animal. Sea lilies rarely 

 have more than 40 arms; some possess only 5. Feather 

 stars may produce up to 200 arms. Most of the many- 

 armed forms are from tropical and subtropical seas. 

 Cold-water or deep-sea forms usually bear 10 arms. 



Modern crinoids gather food by means of a method 

 believed to have been used by all primitive echino- 

 derms. Along a ciliated groove in the upper, oral 

 surface of each arm and its side branches, delicate 

 finger-like tube-feet respond to the arrival of each 

 food particle by bending quickly inward. This action 

 throws the food into the mucus-filled groove, where 

 it becomes entangled and is swept to the mouth. 



Sea lilies remain for long periods, and possibly for 

 life, anchored to the bottom, mostly in water from 

 600 to 15,000 feet deep. Feather stars seldom venture 

 below 4500 feet. Sometimes they swim languidly to 

 the surface, or can be found near shore in shallow 

 water. Feather stars living where sunlight reaches 

 them often have beautiful colors, perhaps exceeded 

 by no other marine animals. Some are bright red, 

 others purple, green, orange, golden, white, black, or 

 even variegated. 



All living crinoids belong to the same order. All go 

 through a swimming embryonic stage that is slightly 

 gourd-shaped, encircled by several rings of cilia, and 

 bearing a little tuft of sensory hairs at one end. As 

 skeletal plates begin to form, the embryo comes to 

 rest on the bottom, becoming attached there. 



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