STARFISH. 



These directions will serve for any species of Asterias. Each 

 student should be provided with a specimen preserved in 

 alcohol or formol and with at least an arm of a dried specimen, 

 as in the dried condition the relations of the plates are more 

 easily made out. There should also be available specimens 

 with the ambulacral system injected. This is easiest accom- 

 plished by cutting off one arm near the disc and injecting 

 through the radial canal. 



EXTERNAL. 



The body is shaped like a five-rayed star; in it distin- 

 guish the central disc and the arms, or rays. In the centre 

 of the disc find the mouth. The side on which it occurs 

 is called the oral surface. Projecting from the oral surface 

 of each arm are the fleshy tube-feet, or ambulacra, and the 

 regions of the oral surface in which they occur are known 

 as the ambulacral areas. Sketch this surface in outline, 

 showing the parts. 



The surface opposite the mouth is the aboral surface. 

 Does it have ambulacra? By feeling and bending see that 

 this surface is composed of numerous hard (calcareous) 

 plates, and that many of these bear spines. On the 

 aboral side of the disc is a rounded body, the madreporite. 

 Is it radial or interradial in position; that is, does it lie in 



the line of a ray or between two rays? Sketch the aboral 



110 



