CLASS ASTEROIDEA 



THE STARFISHES 



THESE animals are named from their star-shaped outline. 

 Some have five long, spreading arms diverging from a 

 small disk, which is hardly more than their point of union ; others 

 have a large body with short arms, which are like angular projec- 

 tions of the body, giving a pentagonal shape. Some genera have 

 a greater number of arms than others ; Solaster has eleven to 

 thirteen, and Heliaster (the sun-stars, found on the west tropical 

 coast) has thirty to forty. 



The mouth of the starfish is in the center of the ventral side. 

 A ventral or ambulacral groove extends through each arm. In 

 these grooves the ambulacra, or tube-feet, are arranged in rows. 

 The ambulacra are hollow, cylindrical bodies, each of which has a 

 sucker at one end, and is connected at the other end with a little 

 globular body, the ampulla. The ventral groove forms a ridge in 

 the body-cavity. It is called the ambulacral zone, and is formed 

 by a double row of elongated plates, which meet and form a 

 raised line along the middle of the inside of the arms, and appear 

 like ribs ; between them lie the ampulla?. These plates are called 

 the ambulacral ossicles. Numerous other ossicles extend over the 

 surface, buried in the integument of the body, making a calcare- 

 ous network, or exoskeleton. The ossicles are connected by muscles 

 so that the animal is not rigid, but is able to bend the body and 

 even creep through comparatively small places. The ossicles are 

 covered with spines of two kinds. The very minute ones are the 

 pedicellarice, and have a jointed end which opens and shuts like a 

 pincers. Their principal use seems to be to remove waste matter 

 or other substances from the body, keeping it free and clean. The 

 pedicellariae are arranged, in some species, in circles around the 



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