ECHINUS. 207 



find them accompanied by corresponding modi- 

 fications in the processes of developement. The 

 organization of the animals belonging to the 

 lowest division of the series is not sufficiently 

 perfect to afford the means, which are supplied 

 in the higher animals, of removing or modifying 

 the substances that have at any time been de- 

 posited, and suftered to harden. Hence the 

 structures composed of these substances remain 

 unchanged during the life-time of the animal, 

 although they may continue to receive additions 

 of new layers of the same material, deposited 

 upon 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 consequences of this law of increase. As 

 these consequences are important in their rela- 

 tion to the conditions 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 

 altogether upon the relative situations of the soft 

 secreting 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, 



