STRUCTURE. 187 



motive. Though far less numerous than the red corpuscles, 

 yet, as ten thousand are contained in a cubic millimetre of 

 blood a mass less than a pin's head it results that the 

 human body is pervaded throughout all its blood-vessels by 

 billions of these separately living units. In the lymph, too, 

 which also fulfils the requirements of liquidity, these amoeboid 

 units are found. Then we have the curious transitional 

 stage in which units partially imbedded and partially free 

 display a partial unit-life. These are the ciliated epithelium- 

 cells, lining the air-passages and covering sundry of the mucous 

 membranes which have more remote connexions with the 

 environment, and covering also the lining membranes of 

 certain main canals and chambers in the nervous system. 

 The inner parts of these unite with their fellows to form an 

 epithelium, and the outer parts of them, immersed either in 

 liquid or semi-liquid (mucus), bear cilia that are in constant 

 motion and " produce a current of fluid over the surface they 

 cover : " thus simulating in their positions and actions the 

 cells lining the passages ramifying through a sponge. The 

 partially independent lives of these, units is further seen in 

 the fact that after being detached they swim about in water 

 for a time by the aid of their cilia, 



5d. But in the Metazoa and Metaphyta at large, the 

 associated units are, with the exceptions just indicated, com- 

 pletely subordinated. The unit-life is so far lost in the 

 aggregate life that neither locomotion nor the relative motion 

 of parts remains; and neither in shape nor composition is 

 there resemblance to protozoa. Though in many cases the 

 internal protoplasm continues to carry on vital processes 

 subserving the needs of the aggregate, in others vital processes 

 of an independent kind appear to cease. 



It will naturally be supposed that after recognizing 'this 

 fundamental trait common to all types of organisms above 

 the Protozoa and Protopliyta, the next step in an account of 

 structure must be a description of their organs, variously 



