n6 



ANIMAL BIOLOGY 



gill cover. These rays grow from the tongue bone. (Zu, 



Fig. 216. This is a rear view.) 



Watch a live fish and determine how the water is forced 



between the gills. Is the mouth opened and closed in the 

 act of breathing ? Are the openings behind 

 the gill covers opened and closed ? How 



FIG. 217. 



CIRCULATION 

 IN GILLS. 



FIG. 218. NOSTRILS, MOUTH, AND GILL OPENINGS OF 

 STING- RAY. 



many times per minute does fresh water reach 

 the gills ? Do the mouth and gill covers 

 open at the same time ? Why must the water 

 in contact with the gills be changed constantly ? Why 

 does a fish usually rest with its 

 head up stream ? How may a 

 fish be kept alive for a time 

 after it is removed from the 

 water? Why does drying of 

 the gills prevent breathing ? If 

 the mouth of a fish were propped open, and the fish re- 

 turned to the water, would it suffocate ? Why, or why not ? 



e, 



FIG. 219. GILL OPENINGS OF 

 EEL. 



Food Tube. The gullet is short and wide. The stomach is 

 elongated (Fig. 220). There -TS a slight constriction, or narrow- 

 ing, where it joins the intestine. Is the intestine straight, or does 

 it lie in few or in many loops? (Fig. 220.) The liver has a gall 

 bladder and empties into the intestine through a bile duct. Is the 



