THE EARTHWORM. 43 



studied, they may be dissected away, to disclose the 

 gullet more fully. Review now the whole digestive 

 tube, pharynx, gullet, crop, gizzard, intestine. 



13. Under the intestine is a ventral vessel. Find its 

 side branches. 



14. Cautiously dissect away the intestine and find under 

 it a slender white thread, the nerve cord, having 

 swellings, or ganglia, in each segment. Trace this 

 nerve cord to its posterior end. Then trace it for- 

 ward. Under the anterior part of the pharynx it 

 divides, sending a branch up on each side of the 

 pharynx. These branches unite in a large, double 

 ganglion above the anterior end of the pharynx. 

 This nerve collar is similar to that found in the 

 crayfish and in insects. 



15. The thin outer skin, or cuticle, which easily peels 

 off in alcoholic specimens, was probably noticed in 

 cutting along the dorsal wall. Observe the pearly 

 lustre of the cuticle. Under the true skin is a layer 

 of muscle, with fibres running circularly. Under- 

 neath this is a second layer of muscle, whose fibres 

 run lengthwise. By the contraction of the circular 

 fibres the segments are made narrower and longer; 

 thus the body is extended. If the spines are held 

 pointing backward, the body will be pushed forward. 

 When the longitudinal fibres contract, the body is 

 shortened, and, owing to the direction of the spines, 

 is pulled ahead. 



16. The reproductive organs are alike in all the individ- 

 uals of a given species of earthworms. All lay eggs. 

 There are no males, no females. They pair, each 

 fertilizing the eggs of the other. Animals of this 



