THE CATFISH. 163 



backwards, flat against the sides of the body; pass a rubber 

 band back over the head and around these fins, to keep 

 them so. Keep the fish under water while attempting 

 to depress the pectoral spines, for in air it will keep 

 them rigidly erect. Pass another rubber band about the 

 ventral fins. Then liberate the fish, and watch it. What 

 position does its body assume ? 



Release the paired fins, and fasten down the dorsal and 

 anal fins with rubber bands. Liberate the fish again, and 

 observe how it gets along without the use of these fins. 

 What kind of a course does it take through the water ? 



II. Circulation seen in Fins. Observe that the broadly 

 expanded part of the fin is very thin and transparent, and 

 that it is traversed by minute blood vessels, which appear 

 as fine red streaks. Connecting these that are visible to 

 the unaided eye, are many smaller capillary vessels in 

 which the circulation can be advantageously studied with 

 a microscope, as follows : 



Wrap the fish in a wet towel, leaving the caudal fin 

 exposed, and place it on a low box beside the microscope, 

 with its caudal fin extending across the center of the micro- 

 scope stage. Spread the fin out flat on a glass slip upon 

 the stage, so as to bring a thin portion of it into the field, 

 and examine it with low power. If the fish refuses to lie 

 quietly, pour a little chloroform on the towel near its 

 mouth. 



Observe the conspicuous, dark, irregular pigment cells 

 scattered throughout the epidermis of the fin. 



III. Blood Vessels. The larger ones are of two kinds : 

 (1) arteries, bringing blood out into the fin, and (2) veins, 

 conveying the blood back to the body again. The smaller 

 ones are the capillaries, connecting the arteries with the 

 veins, aiid distributing the blood throughout the tissues of 

 the fin. 



Observe that the blood consists of a fluid plasma, in 



