CHAPTER XXII 

 THE ECHINODERMS 



The Echinodcrmata or " spiny-skinned animals " are repre- 

 sented only by marine species. The common names applied to 

 these animals are starfishes, brittle stars, sea urchins, sand dol- 

 lars, sea cucumbers, and sea lilies. Echinoderms form quite 

 a conspicuous part of the fauna of the seashore, but they are 

 very seldom seen inland except in museums and curio cabinets. 

 Their structure is very complex and very different from that of 

 other animals, so much so that the term " aberrant " is often ap- 

 plied to the group. It will hardly pay us therefore to use much 

 space in describing them or to spend much time in their study. 



Symmetry. — The most notable thing about the echinoderms 

 is their symmetry. All of the animals that we have studied 

 thus far are bilaterally symmetrical. Animals are either sym- 

 metrical or asymmetrical, and the symmetrical animals are 

 either bilateral or radial. 



The bodies of bilaterally symmetrical animals are so con- 

 structed that the chief organs are arranged in pairs on either side 

 of an axis passing from the head or anterior end to the tail or 

 posterior end. There is only one plane through which their 

 bodies can be divided into two similar parts. An upper or 

 dorsal surface and a lower or ventral surface are recognizable, 

 as well as right and left sides. Bilateral symmetry is charac- 

 teristic of the most successful animals living at the present time, 

 including all of the vertebrates and most of the invertebrates. 



The starfishes and other echinoderms are built on an entirely 

 different plan. Their bodies are made up of similar parts that 

 radiate from a central axis ; that is, they are radially symmetri- 

 cal. These parts number in echinoderms either five or a multiple 



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