PHASCOLOSOMA. 107 



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 cwlomic 

 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 ? 



Circulatory System. This system is very difficult to ob- 

 serve. Dorsal and ventral blood-sinuses are present, and com- 

 municate anteriorly by a circular sinus. A blood-sinus, purplish 

 red in living specimens, occurs, as an irregular tube, along the 

 anterior portion of the esophagus and intestine. 



Excretory System. Find a pair of brown nephridia, an 

 inch or more in length. Cut off a nephridium (from the fresh 

 worm) as close as possible to the body-wall, and examine it 

 under a microscope. Near the cut (the attached) end find the 

 ccelomic opening or nephrostome. Is it ciliated? 



Reproductive System. The sexes are separate. Oogonia 

 and spermatogonia are detached from the ccelomic epithelium, 

 at the points where the ventral retractor muscles are attached 

 to the body-wall. These cells become detached and mature 



