120 ANNELIDA 



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 coelom. 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 mesenteric thread which runs 

 through the axis of the intestine spiral. Where is it at- 

 tached? 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 in- 

 trovert are there? How is the mechanism of protrusion of 

 the introvert to be explained? 



Circulatory System. — This system is very difficult to ob- 

 serve. Dorsal and ventral blood sinuses are present, and 

 communicate 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 ex- 

 amine it under a microscope. Near the cut (the attached) 

 end find the coelomic opening or nephrostome. Is it ciliated? 



Reproductive System. — The sexes are separate. Oogonia 

 and spermatogonia are detached from the coelomic epith- 

 elium, at the points where the ventral retractor muscles are 

 attached to the body wall. These cells become mature while 

 floating in the coelomic fluid. They pass out through the 

 nephridia, which function as gonoducts. 



Nervous System. — Does the ventral nerve cord seem to be 

 double? Is it ganglionated? Does it give off lateral nerve 

 branches? Trace the circumesophageal connectives to the 



