CHAPTER XVIII 

 PHYLUM ECHINODERMATA 



The name of this phylum means " spiny skin," and is 

 fitting for most representatives. Echinoderms are radially 

 symmetrical, and usually have five antimeres; an anus is 

 present in some but non-functional or absent in others ; the 

 ccelom is large and well developed. The distinctive feature 

 of echinoderm structure is the ambulacral, or water-vas- 

 cular, system which serves primarily for locomotion. There 

 are five classes: (1) Asteroidea, starfishes; (2) Ophiuroidea, 

 brittle-stars, or snake-stars; (3) Echinoidea, sea-urchins; 

 (4) Holothuroidea, sea-cucumbers; (5) Crinoidea, stone- 

 lilies. 



CLASS 1. ASTEROIDEA 



The starfishes all have the chief axis short, and the dis- 

 tance from the mouth to the opposite pole of the body is 

 accordingly abbreviated. The central portion of the body, 

 or disc, is not sharply set off from the arms, but grades 

 rather gradually into them. Most starfishes have five arms, 

 or rays, but there are several species which have more; the 

 number sometimes reaching as high as forty. The common 

 starfish along the seashore on the Atlantic coast of the 

 United States is Asterias forbesi (Desor) Verrill, and it will 

 be discussed in detail as an example of the class. 



' THE STARFISH 



Self-maintenance. Asterias (Fig. 80) moves about by 

 means of the tube-feet, of which there are four rows extend- 

 ing from a longitudinal groove along the underside of each 

 arm. A tube-foot has an adhesive disc at its tip and is con- 



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