182 



Practical Zoology. 



disk is the large, thin-walled stomach. Examine this organ care- 

 fully. Pass a blunt probe into the mouth and explore its interior. 



7. Observe the large lobes of the stomach extending a short 

 distance into the rays ; lift one of these lobes and trace the thin 

 retractor muscles of the stomach to the sides of the ridge in the ray. 



In the live starfish the stomach is often found protruded and 

 surrounding a mussel or an oyster ; after digesting and absorbing 

 its soft parts the stomach is retracted. 



8. Turning to the ceca of the anterior ray, trace them toward 

 the stomach ; find the union of their tubes and the entrance of 

 their common duct into the stomach. Observe the place where 

 this tube enters the stomach, in reference to the corresponding 

 lobe of the latter. 



Carefully cut the mesentery along the aboral wall and wholly 

 free the ceca of this ray from all attachment above. Note that 

 the mesentery is double. 



9. Hold the starfish inverted and pour water through the mouth 

 into the stomach to show its shape. 



10. In the other two rays which have been opened, cut 

 across the common ducts of the ceca close to the stomach, and 

 leave them attached to the aboral walls. 



11. Find the extremely short intestine connecting the stomach 

 with the upper wall of the disk, near the junction of the extensor 

 muscles of the rays. Find, also, some small branched append- 

 ages of the intestine. The anal opening is minute. 



12. Sever the intestine close to the aboral wall, cut across 

 the disk close to the madreporic body, and remove entirely the 

 roof of the disk and the three rays. 



Make a drawing of the organs now exposed, showing the 

 ceca in one ray, the reproductive bodies in another, and the 

 ampullae in the third. 



13. Thoroughly examine the stomach, and remove it after 

 cutting across the short esophagus. 



14. The S-shaped stone canal may now be seen passing down- 

 ward from beneath the madreporic body. 



