368 



ZOOLOGY. 



the rows of teeth examined. The thoracic and abdominal portions of 

 the nervous system, wliich lies loosely on the floor of the body, can be 

 readily found by raising the alimentary canal ; but the brain and infra- 

 cesophageal ganglia can best be detected by a longitudinal section of 

 the head. The ovaries always lie above the intesiine, and the two 

 oviducts unite below the nervous cord to form the common duct which 

 opens on the ventral side of the third segment in front of the anuB, 

 which is situated dorsally. Insects should be dissected in a shallow 

 pan lined with wax or cork, and the parts floated out ; fresh specimens 

 are desirable. The body may also be dissected, each segment with its 

 appendages being separated and glued in their true sequence to a card. 

 By simply dissecting an insect in this way, the student will acquire a 

 valuable knowledge of the external structure of insects. 



nr«gon-fly (Dlplax ElieaX 



