104 ANNELIDA. 



4. Four apertures with thickened margins arranged in the 

 form of a square between the thirty-ninth and fortieth and 

 fortieth and forty-first rings. These are the apertures of the 

 mucous glands. 



5. The external apertures of the nephridia about half-way 

 between the median line and the margins, on the posterior edge 

 of the last ring of a somite. The spots are light colored and 

 elliptical. These are important landmarks in determining 

 the boundaries of somites. Some of the anterior and posterior 

 somites do not bear them. 



6. The posterior sucker. Its size, shape, and structure. 



Preserved specimens if very hard should be placed in water 

 some time before dissection. Cut through the body wall along 

 the mid-dorsal line, being careful not to cut underlying organs. 

 Lift up the flap of integument and cut the connective tissue 

 loose so it may be turned and pinned back under water. Work 

 forward and backward from the middle of the back. 



1. The digestive tract consists of a buccal pouch, pharynx, 

 stomach, and intestine. 



2. The pharynx is thick walled, elongated, and bound to the 

 body wall by radiating muscle-fibers. What is their function? 

 The pharynx is muscular and is provided with bands of longi- 

 tudinal and with circular muscles. What function is performed 

 by these fibers? 



3. The stomach joins the pharynx, is large, and has divertic- 

 ula that nearly fill the body cavity. It occupies a considerable 

 portion of the length of the body. How many pouches has it? 

 Do the pouches bear any relation to somites? The posterior 

 end of the stomach narrows and projects into the intestine. 



4. The intestine is enlarged a little at its anterior end and 

 tapers to a slight dilatation, just behind it, that is sometimes 

 called the colon. From this a short rectum runs to the anus. 



Make a sketch of the digestive tract. 



5. Open the digestive tract, wash it out, and examine with a 

 lens. Especially study the pharynx and see how the sucking 



