58 The Under- Water World 



mobility, but above all, have developed 

 an enormous capacity for growth and 

 expansion. The cells constituting these 

 animals, whilst multiplying apace, appor- 

 tioned to themselves an immense variety 

 of occupations, some forming armour, 

 others providing motive and feeding 

 powers. 



Living Echinoderms are classified as 

 (i) Sea Urchins, (2) Starfishes, (3) Brittle 

 Stars, (4) Sea-Lilies, (5) Sea Cucumbers. 

 A number of other divisions flourished in 

 prehistoric times, and are of interest 

 owing to the light they shed upon 

 existing forms. 



The Sea Urchins (Echinoidea) are con- 

 structed on the same plan as are the 

 starfish, but with the rays joined at their 

 tips, and cemented together at their 

 edges. They are generally more or less 

 spherical in shape, but vary from the 

 globular to the discoid. Mouth and vent 

 are always separate, but their positions 



