62 LABORATORY EXERCISES 
prostomium. The aniis is at the posterior end of the body. Com¬ 
pare the dorsal and ventral sides of the body. How did you find 
that they differed in the living specimen? With a lens examine 
the small bristles which lie along each side of the ventral surface. 
What is their use? Note carefully the covering of the earthworm. 
What is its nature? Draw an earthworm, showing its external 
structures. 
III. Internal Structure. (Use prepared museum dissections or 
specimens previously dissected and pinned out by the instructor. 
Compare with Waggoner, Fig. 178.) 
1. Note how the body cavity, the coelom, is divided by thin 
walls into chambers which correspond to the segments shown 
externally. 
2. The Digestive System. The digestive tube runs straight 
through the body from the mouth to the anus. The mouth cavity 
is a thin-walled, sac-shaped structure into which the mouth opens. 
This cavity leads into the pharynx, a region with muscular walls. 
Back of the pharynx is the narrow esophagus. Next come the large 
thin-walled crop and, following it, the thick-walled, muscular 
grinding organ, the gizzard. The remainder of the digestive tube 
is the intestine. Make a drawing of the digestive system, labeling 
its parts. 
3. The Circulatory System. Along the dorsal surface of the 
digestive canal is a slender dark tube, the dorsal blood vessel. In 
the anterior region this divides into branches, the '' hearts,” which 
pass around the digestive system to the ventral blood vessels. These 
hearts ” contract and send the blood into the ventral vessel, 
where it flows in a posterior direction. The contractions of the 
dorsal blood vessel send the blood within it in an anterior direc¬ 
tion. The dorsal and ventral blood vessels and the hearts ” 
constitute the main part of the circulatory system. Make a dia¬ 
gram showing the general plan of the circulation. 
4. The Respiratory System. The exchange of gases necessary 
in respiration occurs throughout the entire body surface. There 
are no special organs for this process. 
