PHASCOLOSOMA. 87 



of sea-water you will find it is rather soft, but becomes turgid 

 immediately upon being touched. How is this accomplished? 



2. Examine a living animal in a dish of sea-water. The 

 anterior portion of the body, the introvert, is drawn in, but may 

 occasionally be extended, when it will be seen to bear at the 

 anterior extremity a crescentic tentacular fold, which partly 

 surrounds the mouth. 



3. Compare with a preserved specimen which has been killed 

 with the introvert extended. 



Make drawings showing the animal with the introvert pro- 

 truded and with the introvert concealed. 



4. The anus is located on a dorsal papilla, anterior to the 

 middle of the body. Near the anus a pair of lateral papillae 

 mark the position of the nephridiopores. The coiled intestine 

 and brown nephridial tubes can probably be seen through the 

 body-wall. Note carefully the character of the skin. Is there 

 any indication of spines, appendages, or eye-spots? 



For dissection use both fresh and preserved specimens. 

 With scissors open the worm from end to end near the mid- 

 dorsal line, and pin the body-wall out flat. 



5. In opening the fresh worm, note the pinkish coelomic 

 fluid which fills the ccelom. Examine a drop under the micro- 

 scope. What functions has this fluid to perform? 



Alimentary Canal. Trace the alimentary canal (stomach- 

 intestine) from mouth to anus. Do any digestive glands open 

 into it at any point? Note the mesenterial thread which runs 

 through the axis of the intestine spiral. Where is it attached? 

 Does it seem to be contractile in the fresh worm? 



Muscular System. Note the silvery-white longitudinal 

 muscles composing the inner layer of the body-wall. Are they 

 arranged in distinct bands or in a continuous sheet? Remove 

 some of these muscles carefully to expose the layer of circular 

 muscles. How many retractor muscles of the introvert are there? 

 How is the mechanism of protrusion of the introvert to be ex- 

 plained? 



