THE LIVING WORLD 219 



The lungs are two very complexly formed air-bags, 

 while a tube the windpipe descends from the back of 

 the mouth, bifurcates below and then ramifies in each 

 lung, the whole constituting the " respiratory system." 

 The membrane which lines certain parts of this system 

 is coated with cilia * like those of infusoria, or those by 

 which the protococcus effects its movements. 



Each kidney is a rounded mass of minute tubes, which 

 converge to open into a cavity whence a tube descends 

 to the bladder which opens externally by a further 

 canal. 



The skeleton is made up of bones and cartilages, and 

 the parts are mostly capable of being moved one upon 

 another by the intervention of the muscles which are 

 attached to them. These movements are facilitated by 

 the shape of the contiguous surfaces of such movable 

 bones, which constitute what are called articulations or 

 "joints." 



The essential part of the generative system of the 

 male, consists of very minute tubes which form small 

 bodies analogous to the antherozoids of ferns. They 

 are rounded bodies which move by the aid of a single 

 cilium only, and are called spermatozoa. 



The essential generative organs of the female do not 

 consist of tubes, but of a peculiar solid substance con- 

 taining modified cells termed ova. 



The nervous system consists of an immense multitude 

 of cords, threads, and cells containing a peculiar albu- 

 minous fluid, These form the brain, spinal cord, nerves, 

 and small rounded aggregations of nervous substance 

 termed ganglia. Nerves proceed from the nervous axis 

 (brain and spinal cord) to all parts of the body, and 



* See ante, p. 200. 



