208 THE MECHANICAL FUNCTIONS. 



and push it back for a space equal to its own 

 thickness. In process of time, the addition of nu- 

 merous layers having led to successive encroach- 

 ments of the solid substance, the latter will have 

 been displaced to an extent which must sooner 

 or later become sensible. If the soft organs have 

 sufficient room for their expansion, as is the 

 case when they are external to the hard axis 

 of the zoophyte, the growth of that axis may go 

 on without impediment ; and no change need 

 take place in the general figure of the parts, 

 since their relative proportions and situations 

 may be preserved unaltered. But this cannot 

 happen when the new materials are to be de- 

 posited on the internal surface of a membrane, 

 or a shell, which completely encloses the soft 

 parts : for the additions thus made to the thick- 

 ness of the layer must encroach upon the space 

 within ; and, that space being limited, the soft 

 parts contained in it will not merely cease to 

 grow, but will be actually contracted in their 

 dimensions : and if the process of deposition 

 were to go on, the space occupied by the soft 

 organs would at last be entirely filled up with 

 solid matter, and the cavity be obliterated. Ac- 

 cordingly it is necessary, whenever cells, in- 

 tended for the lodgement of soft organs, are to 

 be constructed of hard materials, that the foun- 

 dation of these cells should be laid, and their 

 construction begun, upon a scale of the same 



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