The Starfish. 29 



THE STARFISH 



(Asterias sp.). 



External Anatomy. Note the small, cylindrical body, 

 the radiating arms, the centrally placed mouth, and the 

 red madreporic plate. A vertical plane passing through 

 the madreporic plate and thence across the body and along 

 the median line of the opposite arm will divide the animal 

 into a right and a left half. There is then bilateral sym- 

 metry. 



Do you find specialized spines surrounding the mouth? 

 How many? Are the spines specially developed on other 

 portions of the animal ? Note the ambulacral furrows, 

 grooves, which extend along the oral surface of each arm. 

 Each groove is filled with two zigzag rows of ambulacral 

 suckers. Do you find any structure on the aboral side of 

 the arms that suggests a median radial line? Find the 

 pedicellarice, minute pincer-like organs around the bases 

 of the aboral spines. On what other portions of the animal 

 are they present ? 



Draw the animal as seen from above, and again, as seen 

 from below. 



With a strong pair of scissors remove the entire aboral 

 surface of the animal and without in any way injuring the 

 underlying " soft-parts." 



