368 



ZOOLOG r. 



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 ^ai^ing the alimentary canal ; but the brain and infra- 

 cejophageal ganglia can best be detected by a loiijiitudiual section of 

 the head. The ovaries always lie above iLe iutes.iue, 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 anus, 

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

 pan lined vrith wax or cork, and the parts floated cut ; fresh specimeLs 

 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 wUl acquire a 

 valuable knowledge of the external structure of insects. 



Dragon-fly {Diplax Elisa). 



