CHAPTER XIV 



PHYLUM ECHINODERMATA (Starfish, Sea-urchins, Sanp- 

 DOLLARS, Sea-lilies) 



Laboratory Exercises 



239. Asterias (Starfish). — Both dry and alcoholic, or other- 

 wise preserved, materials shotdd be at hand. 



1. General form. 



Central disc. 



Rays; number, form, size, etc. Compare several in- 

 dividuals. 



Oral surface (contains mouth); aboral surface. Note 

 the general differences between these surfaces both 

 in the arms and the disc. 



The axis of an arm is known as a radius; the space be- 

 tween is interradial. 



Is the body bilaterally symmetrical or radially sym- 

 metrical ? Give the reasons for your conclusion. 



2. External anatomy. 

 Oral surface. 



Mouth : position and surroundings. 



Ambulacral groove: position, relation to the mouth, 



extent. 

 Ambulacral feet: how arranged? Is the foot hollow 



or solid? Pull off one, and examine with lens or 



low power of the microscope. 

 Aboral surface. 



Madreporic body: position (radial or interradial?), 



shape, size, structure. 

 Bivium; trivium (see text, §243). 

 Examine the spines on both surfaces and determine 



the arrangement and shape in different regions. 



How are they fixed to the body ? 

 Pedicellarise (at the base of the spines); papulas (soft 



bodies among the spines). Examine with lens. 

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