440 COLLEGE ZOOLOGY 



relieved, and therefore expands, often forcing the gullet out of 

 the mouth. 



The Excretory System. The kidneys lie just beneath the 

 backbone in the abdominal cavity. They extract urea and 

 other waste products from the blood. Two thin tubes, the ure- 

 ters, carry the excretory 'matter into a urinary bladder, where it 

 is stored for a time and then expelled through the urinogenital 

 opening just posterior to the anus. 



The Nervous System. The brain of the perch is more highly 

 developed than that of Petromyzon or Squalus. The four chief 

 divisions are well marked, the cerebrum, optic lobes, cere- 

 bellum, and medulla oblongata. The brain gives off cranial 

 nerves to the sense-organs and other parts of the anterior portion 

 of the body. The spinal cord lies above the centra of the verte- 

 bral column and passes through the neural arches of the vertebrae. 

 Spinal nerves arise from the sides of the spinal cord. 



Sense-organs. The principal organs of sense are the eyes, 

 ears, and olfactory sacs. The mucous membrane of the mouth 

 is the seat of the sense of taste, but this sense is not well developed. 

 The integument, especially that of the lips, serves as an organ 

 of touch. Lateral line organs are also present, but their function 

 is not certain. 



The two olfactory sacs lie in the anterior part of the skull and 

 open by a pair of apertures in front of each eye. They are not 

 connected with the mouth cavity, and take no part in respiration. 

 The inner surface is thrown up into folds which are covered with 

 sense-cells. Water flows in and out through the external open- 

 ings. 



The ear consists of the membranous labyrinth only. As in 

 Petromyzon and Squalus, the sound waves are transmitted by the 

 bones of the skull to the fluid within the labyrinth. Three semi- 

 circular canals (Fig. 350, ca, ce, cp) are present, and the sac- 

 culus (s) contains concretions of carbonate of lime, called ear- 

 stones or statoliths. The ear is both an organ of hearing and an 

 organ of equilibrium. 



