248 ELEMENTARY BIOLOGY. [CHAP. 



not be regarded as in any way distinctive of the adult worm. 

 It undergoes a periodic enlargement, and may be present in 

 young worms but two inches in length or absent in fully 

 formed ones of six or eight. 



LABORATORY WORK. 



The unnatural displacement of the organs of this animal, 

 resultant upon contraction of the muscles during death 

 under chloroform, may be entirely obviated by killing the 

 worm in alcohol. Let it be immersed in methylated spirit 

 for two minutes, and allowed to remain in running water for 

 half an hour. 



A. General external characters. 



1. Examine the living worm and note 



a. Its shape, elongated. Rounded and pigmented 

 dorsally flattened and whitish ventro-laterally. 

 Anteriorly it tapers off to a point, posteriorly it is 

 flattened and spatulate. 



2. Compare the body of the dead animal, killed as 

 above. Note 



a. Its subdivision by a number of constrictions into a 

 recurring series of body segments or somites. Each 

 of these is again subdivided by a lesser constriction 

 into two zonites. 



The zonites of the genital segments (9 to 15) are 

 frequently more numerous, though they never exceed 

 four in number. 



b The clitellum or cingulum (if present, vide supra), 

 a whitish saddle-shaped enlargement usually re- 



