STUDY V. ECHINODERMATA. Spiny-skinned 

 Animals. 



Central idea: Symmetry with relation to habitat and 

 locomotion. 



1. THE STARFISH. 



A. Symmetry. 



1. Identify a central region and five "arms " or rays. 



2. Are there always five rays ? (Examine several 



specimens.) 



3. Are the rays always of the same size ? Why ? 



4. Is the symmetry of the starfish more like that of the 



hydra or that of the earthworm ? 



5. What will you call this form of symmetry ? (See 



p. 41, 7.) 



B. Surfaces. 



1. Identify the upper (aboral) and the lower (oral) sur- 



face. The aboral surface can be recognized by the 

 presence of a wart-like disk (madreporite). On 

 the oral surface is located the mouth surrounded 

 by a membrane (peristome). 



2. Identify the ambulacral grooves extending along each 



ray on the oral surface. 



3. Observe that the shortest dimension of the animal is 



from the upper to the lower surface. Such an 

 animal is said to be flattened. If the shortest 

 dimension of an animal is from side to side, it is 

 said to be compressed. 

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