INTERNAL ANATOMY OF THE EARTHWORM * (1) 



1. Place a preserved Earthworm, dorsal surface up, in a dissecting pan 

 and fasten it with one pin through the prostomium at the anterior end of 

 the body, and another through the posterior end of the body. With fine 

 scissors cut very carefully through the body wall in the median dorsal 

 line, beginning just posterior to the clitellum. Extend the cut to the 

 anterior end of the animal, being careful to cut only through the body 

 wall so as not to injure the intestine just below. Beginning at the poste- 

 rior end of the cut, spread the cut edges by carefully cutting away the 

 segmental partitions (septa) which attach the body wall to the alimen- 

 tary canal. Pin the cut edges of the body wall to the wax in the pan. 

 Slant the pins away from the specimen so as not to interfere with your 

 study. 



2. Examine the specimen and note : (a) the rather thick body wall 

 consisting for the most part of muscular tissue; (b) the body cavity 

 (coelom) which is divided into a linear series of chambers by the septa ; 

 (c) the alimentary canal running the length of the body through the 

 coelom as a straight tube, and (d) the dorsal blood vessel, generally 

 full of coagulated blood, which runs along the top of the alimentary canal. 

 It receives (i) small segmental branches coming from the body wall and 

 alimentary canal, and (ii) connects, in segments 8 to 12, with five pairs 

 of contractile, thick-walled vessels (aortic loops) . The latter pass around 

 the digestive tract and unite below with the ventral blood vessel. 



3. Examine the alimentary canal and note that it consists of the follow- 

 ing parts : (a) the mouth which opens just below the overhanging prosto- 

 mium; (b) the thick, muscular pharynx extending back to segment 6 

 and attached to the body wall by many fine muscles ; (c) the thin-walled 

 esophagus, largely obscured by (d) the overlying reproductive organs, 

 extending between segments 7 and 14 ; (e) the crop at about segment 14 ; 

 (/) the thick-walled, muscular gizzard at about segment 18; (g) the 

 intestine extending from segment 20 to (h) the anus at the extreme 

 posterior end of the body. 



4. Make a drawing, twice natural size, to show these structures as 

 observed, being careful that each organ is drawn correctly with reference 

 to the segments. 



i B. pp. 84-95. 

 281 



