68 HANDBOOK OF INVEKTEBKATE ZOOLOGY. 



around the shell of a mollusc too large to be taken into 

 the mouth. After eversion these pouches are drawn back 

 mto the body by five sets of stomach retractor muscles 

 (Fig. 30, h). These run out for a short distance into 

 each ray, and are attached to the sides of the vertebral 

 ridge. 



i. Cut the muscular attachments of one of the pouches, 

 and raising it up, notice : — 



1. The oesophagus, a short, cylindrical, longitudinally 

 plicated tube. 



2. The pei'istome, or membrane between the outer end 

 of the oesophagus and the edge of the mouth-pentagon. 



j. Make a sketch to show as many of these points as 

 possible, and indicate the relation between the various 

 regions of the digestive tract, and the axis of the rays. 



k. Make also a diagram of a side view of the digestive 

 organs. 



/. Open the pyloric sac of the intestine and notice 

 the valvular folds which guard the opening of the intes- 

 tine. 



IV. The Reproductive Organs. 



Remove the digestive organs by cutting the retractor 

 muscles and the oesophagus. Notice the in+er-radial septa 

 (Fig. 30, (Z) formed l)y the union of the inter-ambulacral 

 plates of adjacent rays. Notice the membraneous folds 

 which form the inner or free edges of these partitions. 

 In the proximal end of the cavity of each ray, between 

 the vertebral ridge and the inter-radial partitions are the 

 light-colored reproductive organs (Fig. 30, ^). 



There are two of these organs in each ray, and their 

 ducts may be traced into the sides of the inter-radial par- 

 titions. 



V. The Amhulacral or Water Vascular Svstem. 



