either quite smooth or roughened without being spiny. If the name 

 echinoderm can be considered vaguely descriptive, it is because all 

 members of this phylum but one - the sea cucumber - have their 

 skin strengthened by nodules or continuous plates of lime. These 

 nodules do sometimes form low spines on the surface; and the 

 box of limy plates that encloses the body of all sea urchins is in 

 some species armed with spines. 



Can we single out one characteristic that all echinoderms have 

 in common and that at the same time distinguishes them from all 

 other groups — their tube feet, say? The answer is no, which once 

 more points up the difficulty of framing precise rules in biology. 

 We cannot even say that, taken as a whole, these animals have 

 progressed a great deal further than the Porifera (sponges) and the 

 Coelenterata in having muscles that give them freedom of move- 

 ment. The sea lilies, for example, remain fixed on stalks once they 

 pass the larval stage. As their name suggests, they are flowerUke, 

 but they so closely resemble a starfish turned upside down and set 

 at the top of a stalk that there can be no doubt about their family ties. 



Moving about, especially the active movement we call loco- 

 motion, demands legs or feet or some equivalent. Every starfish 

 has hundreds of feet. In a groove along the underside of each arm 

 are hundreds of suckers, each sucker at the end of a short tube, and 

 all the tubes connected with other tubes inside the body. This entire 

 system of tubes is filled with fluid, and the fluid, together with the 

 muscles, enables the starfish to pull itself along over the sea bed. 

 The movements of the tube feet and the inflow and outflow of the 

 fluid that makes them rigid are controlled by a complex system of 

 nerve fibers laid out Uke the wires in a telephone exchange, with a 

 nerve running to each tube foot from the main system. 



The echinoderms have acquired a new importance in the eyes of 

 the biologist within the last twenty-five years or so, for they seem 

 to represent a forward leap in evolution. For a long time now we 

 have taken for granted that life began in the sea. We think that the 

 first living things were simple single-celled plants, and that they 



The echinoderms, here represented by a sea 

 cucumber and starfish, are more advanced 

 than the coelenterates. They have tube feet 

 for moving about, a three-layered body wall, 

 an intestine, and a specialized opening for 

 the ejection of waste. 



85 



