CHORDATES IN GENERAL 



163 



from the blood, waste nitrogen products and excess salts in solution 

 as well as to dispose of excess water. The life history of higher 

 vertebrate individuals includes successive stages as follows : the 

 pronephros, the sole kidney for a time ; followed by the mesonephros 

 which is the dominant functional excretory organ ; and, finally, the 

 development of the metanephros with retrogression of the others. 

 This is an illustration of the Theory of Recapitulation which says 

 that each individual in its development lives through abbreviated 

 stages of the history of the development of the race, since these tran- 

 sitional stages represent adult kidneys for more primitive vertebrates. 





Fig. 54. — Cross section of spinal cord and roots of the spinal nerves, showing a 

 simple reflex circuit. 1, Sensory surface of skin ; 2, afferent nerve fiber with S, its 

 cell of origin, located in the spinal ganglion ; If, cut end of spinal nerve ; 5, efferent 

 nerve fiber ; 6, voluntary muscle ; 7, dorsal root of spinal nerve ; 8, ventral root of 

 spinal nerve; 9, dendrites of motor nerve -^cell body in gray matter of the cord. 

 (From Zoethout, Textbook of Physiology, The C. V. Mosby Company, after Morat. ) 



The nervous system in this type of animal is composed of a train 

 and spinal cord forming the central nervous sy stern; nerves extending 

 to all parts of the body, ganglia which are groups of nerve cell bodies 

 outside the central nervous system, and the sense organs which serve 

 for receiving stimuli are usually grouped together under the name 

 peripheral nervous system. A portion of this latter division, con- 

 sisting of two longitudinal trunks with ganglia distributed along 



