Annulata. 51 



DISSECTION OF THE EARTHWORM. 



Material : dissecting pan, forceps, scissors that cut well at the 

 point, dissecting needles, two dozen pins. Ribbon pins are best 

 for this work. Dissect under an inch of water and renew if it 

 becomes turbid. 



1. Lay the specimen lengthwise on the board, stretch it, and 

 pin firmly at each end, slanting the head of the pin away from 

 the worm. Cut through the skin of the back near the posterior end, 

 and continue the cut forward a little to one side of the middle line. 

 With the forceps lift the edge of the cut and run the dissecting 

 needle along under it to cut the partitions that hold the body wall 

 down. Turn the edges of the body wall out and pin them down, 

 slanting the pins at an angle of about thirty degrees so they will be 

 out of the way. 



2. As soon as the edges of the cut are separated the intestine 

 is seen. It is cylindrical, nearly filling the body cavity. It is 

 usually dark-colored from its contents. 



3. Along the top of the intestine is the dorsal blood tube. 



4. Observe now more closely the partitions which extend from 

 the intestine to the body wall between adjacent segments. Com- 

 pare the positions of the partitions with the external markings and 

 bristles. Are there as many segments as indicated by the external 

 appearance ? 



5. Continue the cut to the anterior end, being careful not to 

 cut into the intestine, especially in the part anterior to the girdle. 

 Pin well out, and free the intestine by drawing the dissecting 

 needle along its sides to cut the partitions. 



6. In the region of the tenth segment are several pairs of white 

 bodies, the sperm sacs. 



7. Alternating with these are several red masses. They are the 

 aortic arches, which spring from the dorsal blood tube and arch 

 around on each side to join the ventral blood tube beneath the 

 digestive tube. In some earthworms each aortic arch has several 

 enlargements, making it resemble a necklace. The enlargements 



