EARTHWORM 63 



tral surface is included, showing the somites, setae, clitellum, and 

 reproductive pores. Number each segment. 



II. INTERNAL ANATOMY 



The method of opening the body wall will be demonstrated 

 to small groups in the laboratory. In dissecting the earthworm it 

 is necessary to split open the body wall so it may be pinned out 

 flat, thus revealing the internal organs. In the region just behind 

 the clitellum insert one point of the dissecting scissors and cut 

 forward along the mid-dorsal line of the body to about the sec- 

 ond segment. While making this cut hold the one point of the 

 scissors just inside the body wall stationary, moving only the free 

 blade of the scissors. This will avoid injury to the internal 

 organs. 



Note that the space inside the body wall is divided into small 

 chambers by partitions called septa. With what do these septg, 

 correspond externally? Hold the worm in your left hand and 

 with a dissecting needle note that these septa break away fairly 

 readily. With the needle pressing outward from the cut against 

 the body wall run it along the body until the septa have been 

 broken along both sides of the body. Now with pins open the 

 body wall out flat against the wax in the bottom of a dissecting 

 pan, allowing the heads of the pins to point away from the body 

 of the worm at a broad angle so as to be out of the way during 

 later dissection and examination. 



CIRCULATORY SYSTEM 



1. The dorsal vessel may be seen as a small dark tube which 

 runs along the dorsal surface of the digestive tube. How far 

 forward does it extend? 



2. Large lateral branches, "hearts," pass around esopha- 

 gus. Find at least four pairs. Draw a pair in optical vertical 

 section to show their shape. 



3. At demonstration table examine a piece cut through the 

 body in the region of the hearts to see connection with ventral 

 vessel. 



