CLASS PISCES 



357 



open into the intestine and serve to increase its capacity, taking the 

 place of the spiral valve of the dogfish shark. 



The circulatory system includes a heart which lies in a sac called the 

 pericardium lying below the pharynx. The pericardial cavity represents 

 a portion of the coelom. 



Olfactory lobes 



Roof of cerebrum , poirf 

 between removed 



Cerebrum 

 Pineal body 



Optic lobe 



Cerebellum 



Spinal cord 



Fig. 252. — ^Dorsal view of the brain of a teleost fish, a salmon. {From a Ziegler model, 

 after Wiedersheim.) The cerebellum conceals the medulla in this view. The roots of the 

 cranial nerves are marked by roman numerals. 



The respiratory system consists of four pairs of gills supported by 

 an equal number of bony arches. Each gill bears a double row of gill 

 filaments abundantly supplied with capillaries. The gills are protected 

 externally by the opercula. Internally they are safeguarded by gill 

 rakers from injuries which might be caused by solid particles carried into 

 the pharynx with the water. The gill rakers are projections from the 

 gill arches and are supported by spinelike bones. 



The excretory system consists of paired kidneys, lying just below 

 the backbone in the coelomic cavity, which are mesonephroi. From 



