102 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



fusion of two originally separate cords (as in Planaria) , but the double character 

 appears externally only in the anterior part of the system. Trace the nerve 

 cord anteriorly. Loosen the pharynx and turn it forward. Find underneath 

 it the anterior termination of the ventral cord in a large double mass, the sub- 

 esophageal ganglia. Locate the brain on the dorsal side of the buccal pouch and 

 cut away the pharynx and as much of the buccal pouch as you safely can, leaving 

 the brain in place. Note the two cords which extend dorsally from the sub- 

 esophageal ganglia to the brain, forming a circle through which the buccal pouch 

 passes. These cords are called the circumesophageal connectives. Examine the 

 brain and note that it consists of two distinct lobes, the supra-esophageal ganglia. 

 Find the nerves running forward from the brain to the prostomium and from the 

 connectives to the ventral portions of the first segments. 



Draw the nervous system. 



Strip off a piece of the ventral nerve cord, preferably near the anterior end, 

 mount it in water on a slide and examine with the low power. Find out how 

 many lateral nerves pass out from each ganglion and how many arise between 

 the ganglia, and put this in your drawing. You will probably notice the sub- 

 neural blood vessel and its branches on the ventral surface of the cord. Do not 

 confuse these with nerves; they are generally yellowish and hollow, while the 

 nerves are white and longitudinally striped. 



g) Cross-section of the earthworm: Make a straight, clean cut across the earth- 

 worm in the center of a segment, that is, just halfway between two rings. Use 

 the posterior part of the worm which has not been opened. Make another cross- 

 cut near the first one so as to separate off a small section of the worm. Wash 

 out the contents of the intestine in the piece and place it under water with the 

 first cut surface up. Examine with a hand lens. Identify in the body wall 

 the outer white epidermis, and under this the thicker greenish layer composed of 

 longitudinal muscles. Observe in this the four pairs of setae projecting inward 

 dividing the longitudinal muscle coat into bands. Of these identify the dorsal 

 bands, extending from the median dorsal line to the lateral setae, the lateral 

 bands between the lateral and ventral bundles of setae, and the ventral band 

 across the ventral side. Slender bands also exist between the two members of 

 each pair of setae. In the intestine identify the typhlosole. Between the intes- 

 tine and the body wall in the coelome note the long white nephridia, opening to 

 the exterior just outside the ventral setae. In the median ventral line above 

 the ventral muscle band will be found the white section of the nerve cord. Gently 

 lift out the nephridia and observe the septum stretching across the coelome. 



Make a drawing of the cross-section. 



3. Microscopical structure of the earthworm. Examine slide "Lumbricus" 

 (Hegner, Fig. 85, p. 166). 



a) General appearance of the cross-section: Examine with the low power and 

 identify the thick body wall, the coelome between the body wall and the intes- 



