CHAPTER 28 



THE PHYLUM ECHINODERMATA AND 

 RELATED GROUPS 



Like both the arthropods and moUuscs, the echinoderms are a 

 highly successful group ; however, unlike the members of these two 

 phyla, the success of the echinoderms is confined to the marine habitat. 

 None has successfully invaded either fresh water or the land. 



The body plan of the echinoderms is totally unlike that of any of 

 the other higher invertebrates. The adult is radially symmetrical and 

 gives no clue as to the true relationships of this phylum. The larvae, 

 however, are quite unlike the adults and show that the echinoderms are 

 bilaterally symmetrical animals which have secondarily become radially 

 symmetrical. Further studies of the embryology of these echinoderms 

 show that they are unrelated to the annelids, molluscs, and arthro- 

 pods, rather their relationships are with the chordates. 



The skeletons of the echinoderms are familiar objects to the bi- 

 ologically minded beachcomber. Starfishes of many forms and color, 

 sand dollars, sea urchins — all are left stranded on the sands and in 

 tide pools by the receding tides. Many of the starfishes are among 

 the most colorful of the marine animals; particularly brilliant are the 

 multicolored starfishes and serpent stars of the subtropical waters. The 

 long-spined sea urchins add color to the tropical tide pools, while 

 the sand dollars and sea cucumbers can be found in abundance in the 

 shallow waters. Not only are the echinoderms abundant in shallow 

 waters, but they also occur at the greatest depths. Although there are 

 some 5,000 known species living today, they were even more abundant 

 and varied in the seas of former geologic eras. 



Perhaps a large part of the success of this phylum can be attrib- 

 uted to their possession of a heavy calcareous skeleton covered with a 

 thin epithelium. Most members of the phylum have numerous projec- 

 tions and spines from which the name of the phylum is derived. 



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