THE PERCH 39 



opening of the opercle on each side, close ; at the expiration the 

 oral valves close and the branchiostegal valves open, allowing 

 the respiratory water to pass out through the gill clefts. 



Observe carefully the form and arrangement of the gill arches. 

 Note the gill rakers, the short, spiny projections on the gill arches 

 which prevent solid substances from passing out through the 

 gill clefts. Cut out a gill arch and examine the gills on it. Ob- 

 serve that a double row of gill filaments is present. Study care- 

 fully the arrangement of these filaments with reference to the 

 gill arch and with reference to each other. In fishes the gills are 

 outgrowths of the wall of the pharynx. In the ventral wall of the 

 mouth is the tongue ; note the relation of the tongue to the gill 

 arches. 



Exercise 4. Draw a sketch of the mouth and pharynx, showing both 

 dorsal and ventral surfaces with the features above described. 



Exercise 5. Draw a diagram of a gill arch with its two rows of gill 

 filaments. 



Study the remainder of the digestive tract. Observe the short, 

 wide oesophagus, which joins the pharynx with the stomach. Turn 

 the left lobe of the liver to one side and observe the anterior or 

 cardiac end of the stomach. Note the shape of the liver, and the 

 mesentery which attaches it to the anterior, abdominal wall ; cut 

 this mesentery. Note the mesentery which joins the stomach with 

 the ventral wall of the air bladder, and cut it. Note that a part 

 of the intestine lies free and is not attached to the body wall by a 

 mesentery. Cut the cesophagus and remove the entire digestive 

 tract from the body, retaining, however, its posterior attachment 

 at the anus. 



Study its various parts. The stomach has three distinct re- 

 gions: an anterior region, which^extends straight back from its 

 cardiac portion and ends posteriorly in a blind sac; a posterior 

 region, which leaves the anterior region at right angles near its 

 middle and extends to the beginning of the intestine; and the 

 pyloric appendages, three long, cylindrical blind sacs. Just back 

 of these appendages is a slight constriction which marks the 

 pyloric, or hinder, end of the stomach. 



