248 ELEMENTARY BIOLOGY. [CHAP. 



initiative in the appearance of these mesoblastic somites ; from 

 the central cavities ultimately developed within them the 

 body-cavity is derived, while their walls, coming into 

 apposition antero-posteriorly, give rise to the mesenteric 

 septa. 



The nervous system arises by a series of thickenings of the 

 investing epiblastic layer, and the supra-intestinal vessel 

 when first formed is paired. 



The clitellum, to which reference has been made, under- 

 goes 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. 



Displacement of the organs may be entirely obviated by 

 immersing the worm in methylated spirit for two minutes, 

 and allowing it to remain in running water for half an hour. 



A. General external characters. 



1 . Examine the living worm and note — 



a. Its shape. 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 

 subdivided by a lesser constriction into two zonites. 



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

 frequently more numerous. 



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

 a whitish saddle-shaped enlargement usually re- 



