336 MANUAL OF ZOOLOGY. 



animals, and their function is mainly to excrete water, and the 

 nitrogenous substance known as urea. In the majority of 

 cases the fluid excreted by the kidneys is conveyed to the ex- 

 terior by means of two tubes known as the ureters, which 

 empty themselves into a common receptacle, the urinary 

 bladder. In some cases, however, the ureters open into the 

 termination of the alimentary canal (rectum). 



The nervous system of Vertebrate animals usually exhibits a 

 well-marked division into two parts the cerebro-spinal system, 

 and the sympathetic system. The cerebro-spinal system of 

 nerves constitutes the great mass of the nervous system of 

 Vertebrates, and usually exhibits a well-marked separation 

 into spinal cord (myelon) and brain (encephalon). The pro- 

 portion borne by the brain to the spinal cord differs much in 

 different cases ; and in the Lancelet a brain can hardly be said 

 to be present at all. As already said, the brain and spinal 

 cord are always completely shut off from the visceral cavity, 

 and they are placed upon the dorsal surface of the body. The 

 nerves given off from the cerebro-spinal axis are symmetrically 

 disposed on the two sides of the body, and they are mainly 

 concerned with the functions of "animal " life that is to say, 

 with sensation and locomotion. The sympathetic system of 

 nerves is unsymmetrically disposed to a greater or less extent, 

 and presides mainly over the functions of " organic," or " vege- 

 tative " life, being mainly concerned with regulating the func- 

 tions of digestion and respiration, and the circulation of the 

 blood. In its most fully developed form it consists of a double 

 gangliated cord placed in the visceral cavity on the under sur- 

 face of the spine, and of a series of nervous ganglia, united by 

 nervous cords, and scattered mainly over the great viscera of 

 the thorax and abdomen. 



The organs of the senses are well developed in the Vertebrata, 

 and those appropriated to the senses of sight, hearing, smell, 

 and taste are protected within bony cavities of the head. The 

 perfection of the senses differs much in different cases, but they 

 are probably never wholly wanting in any Vertebrate animal. 

 There are cases in which vision must be of the most rudimen- 

 tary character ; but even in these cases it is probable that there 

 is a perception of light, even if there is no power of distinguish- 

 ing objects. The only cases in which it would appear that 

 vision is really altogether absent, are those of animals placed 

 under the wholly abnormal condition of spending their exist- 

 ence in darkness (such as the Proteus anguinus of the caves of 

 Illyria). Smell, hearing, and taste are probably rarely, if ever, 

 altogether absent in Vertebrates ; though in many cases their 



