204 PHYLUM CHORDATA 



carefully the arrangement of these filaments with reference to 

 the gill arches 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 its relation 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. 



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 ante- 

 rior, 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 oesophagus and remove 

 the entire digestive tract from the body, retaining, however, its 

 posterior attachment at the anus. 



Study the various parts of the digestive tract. The stomach 

 has three distinct regions: 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. 



The intestine begins at the pylorus and extends to the anus. 

 It is composed of three divisions : the duodenum, which includes 

 the anterior loop of the intestine, between the limbs of which the 

 spleen lies ; the small intestine ; and the rectum. The boundary 

 between the last two divisions is the circular ridge about an 

 inch in front of the anus. The liver is a large gland which com- 

 municates with the intestine by means of the bile duct. This 

 duct emerges from the gall bladder, which lies against the pos- 

 terior surface of the liver, receives a number of branch ducts 

 from the liver, and joins the intestine near the base of the pyloric 

 appendages. A pancreas has not been found in the perch. 



