174 PRACTICAL ZOOLOGY. 



of the eyes; it is the ante-orbital 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 ; do they open into the mouth ? do 

 any of them communicate with each other? 



G. 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 sub-opercle; in front of the opercle, and 

 below and back of the eye, bordering the part known 

 as the cheek, is the pre-opercle. If the margin of 

 this be toothed, it is said to be serrate ; under the 

 pre-opercle, and in front of the lower end of the sub- 

 opercle, is the inter-opercle. 



The thin membrane below the gill -cover is the 

 branchiostegal membrane; the curved bones sup- 

 porting it are the branchiostegal rays ; count them. 

 The narrow part of the body between the branchio- 

 stegal 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 teeth-like 

 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-vessels, 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 ? wh^-* 



