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 the 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?
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 liver large or small? Simple or lobed? The spleen (mi, Fig. [220]) lies in a loop of the intestine. The last part of the intestine is straight and is called the rectum. Is it of the same size as the other portions of the intestine? The fish does not possess a pancreas, the most important digestive gland of higher vertebrates.
Fig. 220.—Anatomy of Carp. (See also coloured figure 4.)
bf, barbels on head (for feeling); h, ventricle of heart; as, aortic bulb for regulating flow to gills; vk, venous sinus; ao, dorsal aorta; ma, stomach; l, liver; gb, gall cyst; mi, spleen; d, small intestine; md, large intestine; a, vent; s, s, swim bladder; ni, ni, kidney; hl, ureter; hb, bladder; ro, eggs (roe); mhe, opening of ducts from kidney and ovary.
Questions: Are the kidneys dorsal or ventral? The swim bladder? Why? Why is the swim bladder double? Does blood enter gills above or below?
The ovary lies between the intestine and the air bladder. In Fig. [220] it is shown enlarged and filled with egg masses called roe. It opens by a pore behind the vent. The silver lining of the body cavity is called the peritoneum.
Is the air bladder in the perch simple or partly divided? In the carp? (Fig. [220].) Is it above or below the centre of the body? Why? The air bladder makes the body of the fish about as light as water that it may rise and sink with little effort. When a fish dies, the gases of decomposition distend the bladder and the abdomen, and the fish turns over. Why?