THYONE BRIAREUS 207 



2. Holding the specimen in your left hand, spread with your 

 fingers the tentacles and make a drawing (oral view) with the 

 mouth in the centre, the two smallest tentacles directed down- 

 ward, and the genital opening in the mid-dorsal line. 



3. Place the specimen again on its right side and make a lon- 

 gitudinal incision, beginning close to the^anus and ending at the 

 base of the tentacles between the smallest and the adjoining 

 larger tentacle (along the edge of the fifth radius). Make a 

 similar incision along the edge of the third radius. Cut the body 

 wall across under the tentacles, and in front of the anus, from 

 one incision to the other and remove the wall, carefully sepa- 

 rating it from the viscera. This will expose all organs with excep- 

 tion of those belonging to the radius which has been removed. 

 Pin down the edge of the body wall. Uncoil the alimentary canal 

 by cutting the mesentery close to the body wall. Cut the mes- 

 entery holding the gonad with its duct and push the gonad out- 

 side of the body cavity, taking care not to tear the duct. Push 

 the water lungs outside of the body cavity. Make a full page 

 drawing showing: outline of the body with tentacles, longitudinal 

 muscles, circular muscles, retractors of the pharynx, cloacal 

 muscles; peripharyngeal calcareous corona, oesophagus, stomach, 

 intestine, intestinal mesentery with the rete mirabile, water 

 lungs, cloaca, gonad with genital duct, ambulacral ring canal, 

 hydrophone canal with madreporic plate, polian vesicles and 

 ampullae. 



4. Put a piece of the body wall of the trivium in a test tube 

 with a 10-20% aqueous solution of caustic potash and boil it 

 over a gas flame until all the tissues have been dissolved. Allow 

 the particles to settle to the bottom. Pour off the liquid and 

 wash the sediment in water. Pick up with a fine pipette some of 

 the sediment and put a drop of it on a slide. Find under mi- 

 croscope a calcareous perforated plate of the ambulacral foot. 

 Make a drawing of it. . 



5. Do the same with a tentacle. 



