SEGMENTED WORMS— THE ANNELIDS 173 



the dorsal vessel, causing them to expand and then force the blood out 

 into the ventral vessel when they contract. The blood flows posteriorly 

 in the ventral vessel which gives off branches that lead to all parts of 

 the body and eventually ends up in capillaries, which are a network of 

 fine blood vessels where the absorption of some products and the giving 

 up of others through the principles of osmosis and diffusion take place. 

 Larger vessels again collect the blood at the other end of the capillaries 

 and take it to the dorsal vessel and the cycle is repeated. Circulation is 

 aided by the movements of the coelomic fluid flowing in the cavity which 

 lies between the intestine and the muscle layer and which is given the 

 name coelom. This fluid is colorless and, therefore, contains no hemo- 

 globin like the blood, but otherwise closely resembles it. It can move 

 freely from one segment to another even though there are internal parti- 

 tions, called septa, which separate the segments, but each septum is per- 

 forated in its ventral portion. 



The excretory system consists of a pair of nephridia in each segment 

 except the first three and the last. Each nephridium consists of a little 

 funnel surrounded by cilia which connects to a coiled tube that runs 

 backward and empties to the outside through a small pore on the ventral 

 surface, but lateral to the middle of the body. Excretory waste is swept 

 up by the cilia from the coelomic fluid and carried down the funnel and 

 into the coiled tube. There additional waste may be absorbed from the 

 blood through the capillaries which line the tube, finally to be emptied 

 to the outside. 



Did you ever "worm" your way along the ground? Our soldiers, 

 during the war, in many instances, probably felt a close kinship to the 

 earthworm as they inched over the ground in worm-like motions to avoid 

 enemy fire. You do not travel very fast, but you eventually reach your 

 destination if you keep at it long enough and you realize the importance 

 of a well-developed muscular system in the earthworm. This system 

 consists of circular and longitudinal muscle fibers which are able to ex- 

 tend and to shorten the animal's body length, respectively. There are 

 four pairs of tiny bristles, the setae, that stick out from each segment. 

 These can be controlled by small muscles to each, although they usually 

 point backward. While in this position the setae prevent the worm from 

 slipping backward, so as it shortens and lengthens its body, it moves for- 

 ward. By simply reversing the direction of the setae, so they point for- 

 ward, the earthworm can move backward by the same body movements. 

 If you have ever tried to pull an earthworm from a hole you realize the 

 great power of traction that these setae give, for you can pull the worm 

 in two without succeeding in your endeavor. If you pull an earthworm 



