LESSONS IN ZOOLOGY. 293 



the body are the #7*77 openings, and the bones covering them are 

 the g7*77 covers. The upper posterior part of the gill cover is the 

 opercle; below it is the subopercle; between the opercle and the 

 eye is the preopercle, which is sometimes serrate or toothed, and 

 below it is the interopercle. Below the gill cover find the bran- 

 chiostegal membrane, supported by curved bones called bran- 

 chiostegal rays. The narrow body in the median line separating 

 the right and left branchiostegal membranes is the isthmus. 



Turn back or cut away the gill cover and examine the gills. 

 Notice the central bony arch, and the fringe of red filaments that 

 makeup each gill. On the front and inner border of each arch are 

 teeth-like bodies, called gill-rakers. The slits between the gills 

 are the gill clefts. A red spot on the inside of the gill cover is the 

 false gill. Below the four gill arches is a fifth arch, called the in- 

 ferior pharyngeal bone, and where the gill arches unite above is 

 the superior pharyngeal bone, each of which bears teeth. 



Study a scale; notice its radiate and concentric markings. A 

 scale with an even posterior border is a cycloid scale, and one 

 with a toothed border is a ctenoid scale. Compare scales from 

 different parts of the fish. Notice a line called the lateral line 

 along the side of the fish. Compare a scale from the lateral line 

 with other scales and note the differences. Note that the arrange- 

 ment of the scales is regular and symmetrical. Count the scales 

 in the lateral line. Count the rows of scales above and below the 

 lateral line on the side of the fish. Remove a scale carefully and 

 notice a thin skin or epidermis covering it, and if we examine a 

 dark spot we find that the coloring-matter is in the epidermis 

 and not in the scale. 



Open the fish by cutting from the anus forward in the median 

 line to the isthmus ; just back of the ventral fins, cut toward the 

 spinal column on each side, so as to open the body cavity fully. 

 Notice a silvery white membrane, the peritoneum, and in the front 

 part, a little to the left, a brownish mass, the liver, and under it 

 the hepatic vein. 



Placing the fish on its right side ; turn down the liver, uncover- 

 ing the stomach. Pass a probe through the mouth and gullet 



