PISCES, TRUE FISH 445 



through the toothed mouth, pharynx, esophagus, stomach, duodenum, 

 ileum, and large intestine during the process of digestion. Teeth 

 are located on the jaws, roof of the mouth, and walls of the pharynx, 

 and are used primarily for holding prey. Gastric glands in the wall 

 of the stomach supply some of the digestive juices. Pyloric caeca 

 which join the anterior portion of the duodenum increase the 

 absorptive and digestive surface in many fish. 



The respiratory system consists of the mouth, gills, and, to some 

 extent, the swim bladder in certain fish. Water is drawn in or 

 inspired through the mouth and forced out or expired through the 

 gill slits. The mouth and pharynx form a water-tight pumplike 

 arrangement with the help of the flaplike oral valves just inside the 

 lips and the IrancJiiostegal memhrane at the margin of the operculum. 

 The exchange of oxygen and carbon dioxide between the blood in 

 the capillaries of the gills and the water occurs as the water passes 

 over the gill lamellae. Oxygen is absorbed by the blood, and carbon 

 dioxide is discharged to the water. 



The circulation in most fish is, in general, similar to that described 

 for the lamprey, except for certain specializations and phylogenetic 

 developments. The system includes the paired anterior and poste- 

 rior cardinal veins meeting in the duct of Cuvier which joins the 

 sinus venosus, the hepatic portal vein leading to the liver, the hepatic 

 vein from liver to sinus venosus, the two-chambered heart with its 

 accessory sinus venosus and lulhus arteriosus, ventral aorta, bran- 

 chial arteries, dorsal aorta, and the various branches. 



Excretion is accomplished by a pair of dorsally located meso- 

 nephric kidneys, each of which is connected by a mesonephric duct 

 to a urin<iry sinus or bladder. This bladder opens to the exterior by 

 an aperture located just posterior to the anus. 



Classification 



Few students of the fish are in complete agreement on the classi- 

 fication of all groups of fish, but a general scheme of the most 

 acceptable plan will be presented. There are something like 20,000 

 species of fish described, of which more than 3,300 species occur in 

 North America. There are two subclasses. An abridged summary 

 of their orders and families is included here. 



