180 Practical Zoology. 



plates are known as the interambulacral plates. Examine these 

 closely for comparison with the sea urchin. 



6. The wartlike elevation on the aboral surface is the madre- 

 poric body. Note that it is situated opposite one of the inter- 

 radial angles. Examine it with a lens. 



7. Make drawings of the oral and aboral surfaces of the 

 starfish. 



ALCOHOLIC SPECIMEN. 



1. Briefly review the points noticed in examining the dried 

 specimen. Bend the rays ; their flexibility is now much less than 

 in life. 



2. Compare the spines of different areas as to their shape, size,, 

 and degree of mobility. 



3. Between the spines are soft, tapering projections, the 

 aboral tentacles. 



4. Observe a circle of projections surrounding the spines ; 

 delicately pinch them with the forceps to determine their con- 

 sistence ; remove some of these bodies to strong alcohol ; mount 

 temporarily in turpentine on a slide, cover, and examine with 

 a low power. There should be distinguished a short stalk bearing 

 a pair of pinchers ; these bodies are the pedicellariae. In the live 

 starfish these pinchers may be seen continually snapping ; they 

 are supposed to serve in removing foreign matter from the 

 body. 



5. The soft, cylindrical projections along the median tract of the 

 oral surface of each ray are the ambulacra or tube feet. Remove 

 one of them and examine it with care. Note the arrangement of 

 the series. 



6. Press apart the tube feet and find running along the median 

 line of the ambulacral groove, a yellowish or whitish ridge, the 

 nerve of the ray. Trace it to the soft membrane bordering the 

 mouth, the peristome, and find the nerve ring around the mouth. 



7. Trace the nerves also to their outer ends and find a reddish 

 or yellowish elevation, the eye-spot, borne at the base of a median 

 terminal tentacle, resembling a tube foot. 



