THiS STAKPISH. 151 



4. Find the radiating nerve in each arm, between 

 the rows of tube feet. Trace it from the circum-oral 

 nerve ring around the mouth , to the eye-spot at the tip 

 of the ray, or arm. 



5. The lower surface is called the oral surface. 

 The upper surface is the aboral surface. Draw the 

 oral surface. 



6. Remove carefully the aboral surface. Find on 

 the inner surface of the part removed, in each ray, a 

 thin band, the extensor muscle. What is its use? 



7. Find the hepatic caeca, which are the large, 

 brown, lobulated organs in each ray. The membrane 

 which holds them in place is the mesentery. Are the 

 hepatic cseca connected with the stomach? 



8. Note the stomach, in the center of the disk. 

 How is it connected with the aboral surface ? Find 

 the retractor muscles, which draw the stomach back 

 into each ray. 



9. Find the reproductive organs, which are along 

 the angle of each ray. There is little difference in ap- 

 pearance between the organs of the male and the fe- 

 male starfish. * 



10. Find the stone canal, which extends from the 

 madreporic body to the circum-oral water ring. From 

 the circum-oral water ring, trace the radial water tube 

 into each arm. 



11. The radial Avater tubes carry water into the 

 ampuUse, or little sacs, which lie in the floor of each 

 ray. What is the relation between the ampullae and 

 the tube feet? Are the ampullae that are near the 

 mouth like the other ampullae? 



12. Examine the plates which constitute the frame- 

 work. 



Facts About the Starfish. 



The starfish represents the branch of Echinoderms. 

 It is named froni a thick skin which is beset with 



