188 THE MECHANICAL FUNCTIONS. 



of removing or modifying the substances that have 

 at any time been deposited, and suffered to harden. 

 Hence the structures composed of these substances 

 remain unchanged during the life-time of the ani- 

 mal, although they may continue to receive addi- 

 tions of new layers of the same material, deposited 

 on their surface by the soft parts in contact with 

 them ; for it is through the medium of the soft 

 parts alone that these materials are supplied. All 

 the solid structures of zoophytes are formed by this 

 process, and they are subjected to all the conse- 

 quences of this law of increase. As these conse- 

 quences are important in their relation to the con- 

 ditions of growth, and to the forms which result, it 

 will be necessary to direct our attention to them 

 more particularly. 



The influence, which this mode of increase by 

 superficial depositions may have in changing the 

 form of the original structure, will depend alto- 

 gether upon the relative situations of the soft se- 

 creting organ, and the hard part on which it is to 

 deposit new layers : for, as every new layer must 

 occupy the situation of the soft organ which has 

 formed it, it must displace the latter, and push it 

 back for a space equal to its own thickness. In 

 process of time, the addition of numerous layers 

 having led to successive encroachments 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 ex- 

 pansion, 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, 



