CHAPTER XIII 

 STARFISHES, SEA-URCHINS AND SEA-CUCUMBERS 



The starfishes, sea-urchins, sand-dollars and sea-cucumbers, 

 branch Echinodermata (Gr. echmos, hedgehog; derma, skin), 

 compose the only branch of animals all of whose members 

 are exclusively marine. Although they are among the most 

 common inhabitants of all sea beaches no species has adapted 

 itself to life in fresh water. Why this is so no one is yet able 

 to explain. Most of them can move about freely but some of 

 the feather stars are attached to rocks or other objects as the 

 polyps are. 



The Starfish. The common five-rayed starfish well illus- 

 trates the general plan of structure of members of this group. 

 The five rays arranged around the central disk illustrate the 

 radial symmetry which is characteristic of the branch. The 

 entire aboral or upper surface, as well as a greater part of 

 the oral or lower side, is thickly studded with the calcareous 

 plates, or ossicles, of the body-wall. These ossicles support 

 many short, stout spines arranged in irregular rows, and 

 numerous pincer-like processes, the pedicellaria. In the inter- 

 spaces between the calcareous plates are soft fringe-like pro- 

 jections of the inner body-lining, the respiratory caeca. A little 

 to one side of the center of the disk is the small striated cal- 

 careous madreporic plate, and a little nearer the center is the 

 anal opening. At the tip of each arm or ray is a cluster of 

 small calcareous ossicles and within each cluster a small speck 

 of red pigment, the eye-spot or ocellus. 



On the oral (under) surface are the centrally-located mouth, 

 and the ambulacral grooves running longitudinally along each 

 ray. In each groove are two double rows of soft tubular 

 bodies with sucker-like tips. These are called the tube-feet, 

 and are organs of locomotion. 



