STAEFISH 89 



The peristome or membrane surrounding the mouth is only 

 imperfectly observable from the oral surface. Note this mem- 

 brane again after the dissection has been made under section 8 

 of the Digestive System. 



The amhulaci^al grooves. One runs outward along the ven- 

 tral surface of each ray. They contain : — 



The amhulacral feet or tuhe feet. Note their terminal 

 suckers. 



10. Draw the oral surface showing details of the disc and 

 one ray. In determining the arrangement of the tube feet pull 

 off the feet from a portion of one groove and note the openings 

 from which the feet protrude. 



II. Internal Organs 



Kemove, under direction, the aboral wall of the anterior ray 

 and the disc, being very careful to leave undisturbed those 

 portions directly beneath the wall of the disc and in the vicinity 

 of the madreporic plate. 



The Digestive System 



Note how the pyloric ceca are attached to the aboral walls of 

 the arms and how the stomach is attached to the aboral wall of 

 the disc. Note further : 



1. Intestine, a very short tube, extending from pyloric sac 

 to the anus. The anus is vestigal and cannot be seen from the 

 aboral surface. 



2. Intestinal ceca opening into the intestine. How many 

 lobes ? It is doubtful if these ceca have any digestive function. 

 Recent experiments have shown that the intestinal ceca pulsate 

 and it is suggested that they may have a respiratory function. 



3. The pyloric cecum is the conspicuous two-branched 

 structure occupying most of the space within each ray. This 



