THE FISH. 75 



effect on the gill-rakers ? The slits between the gills, 

 which allow communication from the mouth to the 

 gill-opening, are the gill -clefts. How many gills are 

 there ? How many gill-clefts ? After this study of 

 the gills in their natural position, remove the fore- 

 most gill, severing it at its upper and lower ends, and 

 note more fully the parts above named, especially the 

 structure and arrangement of the gill-filaments and 

 gill-rakers ; tear away some of the filaments, and find 

 the groove along the posterior, outer border of the 

 bony arch in which run the blood-vessels. Look 

 on the inside of the gill-cover for a red spot — the 

 false gill. 



8. Observe the arrangement of the scales. Pull out a 

 scale and study its shape and markings, the radiat- 

 ing and concentric striae. Compare its inner and 

 outer surfaces, its anterior and posterior margins ; 

 make a drawing of it, naming its parts ; pull out a 

 scale from a black spot ; compare that part of its sur- 

 face which was exposed with the part overlapped by 

 other scales ; scrape the portion that was exposed ; 

 thrust one point of the forceps under the hind edge 

 of a scale, and watch closely this edge, while slowly 

 raising it, to see that a thin skin covers it and passes 

 on to the scale behind. This thin outer skin is 

 chiefly epidermis. In this epidermis lie the black 

 pigment cells which make the dark spots. 



A scale with a smooth hinder border is a cycloid 

 scale; if the hinder portion is toothed or spiny, the 

 scale is ctenoid. 



9. A raised line along the side is the lateral line. Re- 

 move one of the scales on this line, and find what 

 makes the line. Is the line continuous? 



