28 



THE STAR-PISH; Asterias rubens. 



Observe in the living star-fish the natural movements. Notice that locomotion is 

 effected by the pedicels or tube feet on the under (oral) surface of the rays, these 

 pedicels having suckers at their tips, whilst the terminal pedicels of the rays are 

 unprovided with suckers, and act as tentacles. 



Turn the star-fish on its aboral surface and observe how it twists round the tip of 

 one or more of its rays until the pedicels situated there can be attached, the pedicels 

 further back being then applied by a similar and progressive twisting of the ray, 

 until by this spiral-like movement the disc is drawn over. 



A. Make a sketch (natural size) of the slightly convex aboral surface, showing the 

 central pentagonal disc with the five rays radiating from it ; the whitish circular 

 madreporic tubercle placed towards one side of the disc and opposite the point of 

 junction of the two rays which form the bivium ; the minute anal opening placed 

 slightly to the left of an imaginary line from the tubercle to the tip of the 

 opposite ray ; the calcareous ossicles bearing short spines embedded in the soft 

 tissue of the integument, and the short stems bearing pincer-like heads — ^the 

 pedicellarisB. 



Eemove one of these pedicellarise, examine with low power and draw. 



B. Sketch the flat oral surface, showing — 



(a) The large central mouth. 



(b) The five ambulacral grooves running along the oral faces of the rays. 



(c) The two double rows of pedicels nearly filling these grooves. 



(d) At the tip of each ray the small red eye spot. 



C. 1. Eemove and examine under low power one of the pedicels, noting the sucker- 



like tip strengthened by five or six calcareous plates. 



2. Examine also a pedicel from the tip of the ray, and notice that it is not 



terminated by a sucker. 



3. Pull off carefully the pedicels from a portion of one of the rays, and notice 



the radial nerve extending along the centre of the ambulacral groove to 

 end in the eye-spot. 



