gill cover. These rays grow from the tongue bone. (Zu, Fig. 216. This is a rear view.)
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Fig. 217.—Circulation in Gills.
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Fig. 218.—Nostrils, Mouth, and Gill Openings of Sting-ray.
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 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 returned to the water, would it suffocate? Why, or why not?
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Fig. 219.—Gill Openings of Eel.
Food Tube.—The gullet is short and wide. The stomach is
elongated (Fig. 220). There is a slight constriction, or narrowing,
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
