60 NORTHERN FISHES 



Internally, fishes possess virtually the same systems of organs as the 

 land vertebrates. The ^pharyngeal, or throat, cavity opens into the 

 stomach just back of the gills. Owing to the absence of the neck, the 

 esophagus is usually not well defined. The stoTuach is so close to the 

 mouth that it is possible to remove food from the stomachs of many 

 fishes bj' inserting an ordinary pair of pliers down the fish's throat. Most 

 fishes have a simple saclike stomach, but a few, such as the gizzard 

 shad, have highly specialized stomachs. 



The stomach empties into the intestine. Near the union of the intes- 

 tine and the stomach, fingerlike pockets known as pyloric caeca are 

 sometimes present. In some species these pyloric caeca are filamentous 

 and very numerous. In sharks and a few primitive bony fishes, such as 

 the paddlefish, the intestine is a short spiral tube, called the spiral valve. 

 Most modern fishes have a more or less coiled tubular type of intestine, 

 such as is found in the land vertebrates. The intestine discharges waste 

 through the anus, commonly called the vent. Near it are the openings 

 for the excretory and genital diicts. 



Fishes possess a hver, pancreas, and spleen, which in general are 

 similar to those of the land vertebrates. The kidneys are straplike or- 

 gans in the roof of the body cavity, sometimes appearing as dark red 

 streaks below and on each side of the backbone. The ovaries or testes 

 are also located in this same general region. During or just preceding 

 the spawning season these organs become very large, the ovaries often 

 crowding the entire body cavity with thousands of eggs. 



All Minnesota fishes are egg-laying forms. The eggs are laid before 

 they are fertilized, and the male liberates the sperm, or milt, over or 

 near them. Fertilization is accomplished by the sperm's actually swim- 

 ming or floating to the egg and uniting with it in the water. 



The stcim, or air, bladder is a large, tough-walled sac just below the 

 kidneys and is sometimes divided. In a few fishes, such as the dogfish 

 and the gars, this bladder is connected to the throat region and can be 

 used as a lung. Such fishes as gars, bo\yfins. and bullheads gulp air into 

 it. In most fishes the swim bladder does not retain its connection with 

 the digestive system but is a closed sac filled with a mixture of oxygen, 

 nitrogen, and carbon dioxide secreted from the blood. Its chief function 

 seems to be as a hydrostatic organ regulating the buoyancy of the fish. 

 Some fishes can use the oxygen in the swim bladder as a reserve sup- 

 ply for respiration and thus descend for several hours into water defi- 

 cient in oxygen. 



The heart is located far forward, just under the gills. It is composed of 

 a single auricle and ventricle. All blood is pumped directly from the 

 heart into the gills, where it is aerated and then distributed to all parts 

 of the body. 



The brain is small in proportion to the size of the body. A catfish 

 weighing almost as much as a man has a brain not much larger than a 



