THE INNER TISSUES OF ANIMALS. 339 



in the individual and in successive individuals, cannot cease 

 until there is a complete balance between the actions of the 

 changed agency and the changed tissue. 



300. We come now to differentiations among the truly 

 inner tissues the tissues which have direct converse neither 

 with the environment nor with the foreign substances taken 

 into the organism from the environment. These, speaking 

 broadly, are the tissues which lie between the double layer 

 forming the integument with its appendages, and the double 

 layer forming the alimentary canal with its diverticula. We 

 will take first the differentiation which produces the vascular 

 system. 



Certain forces producing and aiding distribution of liquids 

 in animals, come into play before any vascular system exists ; 

 and continue to further circulation after the development of 

 a vascular system. The first of these is osmotic exchange, 

 acting locally and having an indirect general action; the 

 second is local variation of pressure, which movement of the 

 body throws on the tissues and their contained liquids. A 

 few words are needed in elucidation of each. If in 



any creature, however simple, different changes are going on 

 in parts that are differently conditioned if, as in a Hydra, 

 one surface is exposed to the surrounding medium while the 

 other surface is exposed to dissolved food; then between the 

 unlike liquids which the dissimilarly-placed parts contain, 

 osmotic currents must arise; and a movement of liquid 

 through the intermediate tissue must go on as long as an 

 unlikeness between the liquids is kept up. This primary 

 cause of re-distribution remains one of the causes of re-dis 

 tribution in every more-developed organism: the passage of 

 matters into and out of the capillaries is everywhere thus 

 set up. And obviously in producing these local currents, 

 osmose must also indirectly produce general currents, or aid 

 them if otherwise produced. In the absence of a pumping 

 organ, this force is probably an important aid to that 



