PHYLUM ECHINODERMATA 



Class : A steroidea 

 A STARFISH 



Several species of starfishes are common along our coasts, the 

 most familiar being Asterias vulgaris, the common New England 

 form, which is found along the entire Atlantic coast, and Asterias 

 forbsii, which is common south of Cape Cod. They are remark- 

 ably sluggish creatures which live on the sea bottom, moving 

 slowly, often in large numbers, from place to place and feeding 

 on the various mollusks which come in their way. 



Two specimens will be needed for this dissection, a dried one 

 for the study of the hard parts, and one that is fresh or has been 

 preserved in formalin or alcohol for the study of the internal and 

 other soft parts. To prepare a dried starfish the live animal 

 should be placed in fresh water for half an hour. It should then 

 be placed in alcohol for an hour, and then dried thoroughly. If 

 only preserved material is at hand, the animals may be simply 

 dried. The fresh water and alcohol expand the body wall of the 

 animals and prevent it from collapsing after death. 



Study the external characters of a fresh or a preserved speci- 

 men. Observe the color and the flattened, radiate body form. 

 The body is composed of a central dis^ from which radiate five 

 arms, or rays. All these rays are normally of equal length. 

 Specimens are often found, however, in which the length of the 

 rays is unequal. This is due to the fact that starfishes often lose 

 one or more of their rays by accident ; the missing member is soon 

 replaced by a new ray, but while it is growing out it will be shorter 

 than the others. The spaces between the rays are called inter- 

 rays. In the center of the under surface of the disk is the mouth ; 

 hence this surface of the animal is called the oral surface. Its 

 upper surface is called the aboral surface. 



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