72 Practical Zoology. 



immediately in front of the eyes ; it is the anteorbital bone. This 

 and several smaller bones just under the eye are known as sub- 

 orbital bones. 



5. Examine the nostrils in front of the eyes. How many are 

 there? Probe them with a bristle tipped with sealing wax, or 

 with the head (never the point) of a small pin ; do they open into 

 the mouth? Do any of them communicate with each other? 



6. The flap at the side of the head is the gill cover, and the 

 opening back of it is the gill opening. The upper, hinder piece 

 of the gill cover is the opercle ; along its lower posterior border, 

 and rather closely attached to it, is the subopercle ; in front of the 

 opercle, and below and back of the eye, bordering the part known 

 as the cheek, is the preopercle. If the margin of this be toothed, 

 it is said to be serrate ; under the preopercle, and in front of the 

 lower end of the subopercle, is the interopercle. 



The thin membrane below the gill cover is the branchiostegal 

 membrane ; the curved bones supporting it are the branchiostegal 

 rays ; count them. The narrow part of the body between the 

 branchiostegal membranes is the isthmus. 



7. Raise the gill cover and examine the gills ; each gill has a 

 central bony arch ; on the hind and outer border of this arch is a 

 fringe of red gill filaments ; on the front and inner border of the 

 arch are the teethlike gill rakers. Are these alike on all the 

 gills? A red streak along the arch, at the base of the filaments, 

 is made by the blood tubes, which bring the blood to and carry 

 it away from them. 



Thrust a finger into the mouth, and depress the tongue. What 

 effect has this on the gills? What effect on the gill rakers? The 

 slits between the gills, which allow communication from the 

 mouth to the gill openings, 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 foremost 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 



