106 THE BIOLOGY OF THE ANIMAL 



d. The "cross -shaped" ossicles. — "With what do 



they connect ? What sort of spines do they bear ? 



Do these ossicles consist of a single piece ? Do 



you find any openings through them? 



Cut along each side of the ray and remove the aboral 



portion, and notice in the middle of the floor of each ray 



e. The vertebral ridge — How is it formed? Why 



called " vertebral " ? • 

 Remove the top of the disk and note 



/. The moutli-opening. — What is its shape? How 

 formed ? How does it compare in size and shape 

 with the mouth itself ? 



g. The inter-radial partitions. — How many are there? 

 What position have these with reference to the 

 mouth - opening ? Of what are they formed? 

 Look on each side of each partition for a small 

 opening, the inner end of the reproductive ori- 

 fice. Push a fine-pointed bristle into the orifice, 

 and try to find the outlet of the tube. 



h. The chemical composition of the ossicles. — Drop 

 a few of the ossicles into ten per cent, hydro- 

 chloric acid in a test-tube or watch-glass. The 

 formation of bubbles in the fluid shows that the 

 ossicles contam carbonate of lime. Compare with 

 the spicules of Spongilla and GranUa. 



Internal Anatomy. — Either fresh or alcoholic speci- 

 mens may be used. The organs of the latter, though 

 somewhat changed in color, possess the advantage of 

 being toughened, and hence are less easily torn. With 

 a pair of strong scissors make a transverse cut through 

 the roof of the rays of the trivium, near the tip of each. 



