72 LABORATORY WORK. 



it, and follow it forward to the brain, in front of the mouth. 

 How many ganglia do you find in the cephalothorax? 

 Do any show signs of being double? Is the commissural 

 cord single or double in this region? Note how the cord 

 passes the sternal artery. Is there a ring of the nervous 

 system around the oesophagus? Can any of the nervous 

 system be said to be above, or any below, the alimentary 

 canal? From what part do the nerves to the antennae 

 and eyes arise? 



Draw the nervous system from above. 



Beneath the nervous system will be found the ventral 

 artery with its anterior and posterior (abdominal) parts. 

 Trace the blood-vessels from it to the appendages. Note 

 also the blood-vessels in the gills. Sketch a gill showing 

 the blood-vessels found. 



