112 THE BIOLOGY OF THE ANIMAL 



"What becomes of the tentacles ? What lies below 

 this outer membrane ? Putting together the facts 

 just learned, what can you say is the structure of 

 an aboral tentacle ? What is the structure of the 

 membrane containing the ossicles ? Does this mem- 

 brane extend down between the ossicles, or does 

 it merely cover their upper and under surfaces ? 



F. — The Nervous System. 

 With a lens carefully examine the lower surface of the 

 circum-oral water-tube for a thickened ridge, the 

 circum-oral nerve -ring, running around its 

 outer surface. Running from this ring just be- 

 low, i. e., outside, the radial water-tube in each 

 ray is a radial nerve which extends to the tips 

 of the ray and ends at the eye. The relation of 

 this nerve to the neighboring parts is much bet- 

 ter seen in cross-sections of the ray prepared for 

 microscopic examination. 



PHYSIOLOGY 



Nearly all of the following work must necessarily be 

 done on the living animals. They should be studied 

 among their natural surroundings whenever possible, 

 otherwise in an aquarium well supplied with an abun- 

 dance of running sea-water. 



Experiments requiring the removal of portions of the 

 body of the living animal probably cause little if any 

 pain, since the starfish frequently parts with one or 

 more of its rays voluntarily. 



A. Movements. — What sort of motion has the starfish ? 

 Plow rapidly can it move? What are its organs 



