IS6 VERTEBRATES 



going to each gill. After passing through the gills they 

 reunite to form the dorsal aorta which passes backward just 

 beneath the spinal column, and after going over the entire 

 body returns to the gills. This is called a single circulation. 

 Draw heart (Xs)- Make a diagram of the circulatory 

 system. 



(id) Open the mouth. See the thin membrane form- 

 ing the floor of the mouth on each side of the tongue. Cut 

 through this thin membrane close to the inner border of the 

 lower jaw. Continue the cutting backward on each side 

 of the gill cover and the branchiostegal membrane and 

 wholly separate them. Turn back this flap and again ex- 

 amine the gill. Note the joints in the gill arches. Where 

 the gills unite above and below are the pharyngeal teeth. 

 The bones supporting these teeth are the pharyngeal bones. 

 They represent a fifth gill arch. Where is the gill? 



(ii) Examine the Nervous System. 



a. With a scalpel open the skull from the dorsal side. 

 Note the successive coverings of the cranium (i) a tough 

 skin, (2) a cartilege, thicker, (3) a gray soft membrane 

 of cellular tissue, still thicker. After removing the last the 

 brain is exposed. Observe the color and relative size of 

 parts. 



Notice the general shape and structure of the brain. How 

 many lobes do you find? Are the lobes in pairs? 



Trace the brain back to the spinal cord, the white exten- 

 sion from the base of the brain. 



Observe the largest lobes of the brain. These are the 

 optic lobes. What shape are they? 



In front of these are the cerebral lobes called cerebral 

 hemispheres. Note their shape, size, location, etc. Lift 



