STAEFISH: LABORATORY WORK. 



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. Running along 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 sur- 

 face, and draw a line through it dividing it into symmetrical 

 halves. How many such lines can be drawn ? The arm 

 opposite the madreporite is known as the anterior ray.* 



With the needle demonstrate that the calcareous plates 

 are not on the outside. What covers them ? Are the 

 spines movable on the plates ? Scattered over the aboral 



* The reasons why this is called anterior rather than posterior 

 cannot be worked out on the forms selected for dissection, but can 

 only be seen by a comparison with the heart-urchins (Spatangoids), 

 etc. 



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