ECHINUS. 189 



since their relative proportions and situations may 

 be preserved unaltered. But this cannot happen 

 when the new materials are to be deposited on the 

 internal surface of a membrane, or a shell, which 

 encloses the soft parts : for the additions thus made 

 to the thickness 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 dimen- 

 sions : 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. Accordingly it is necessary, 

 whenever cells, intended for the lodgment of soft 

 organs, are to be constructed of hard materials, that 

 the foundation of these cells should be laid, and 

 their construction begun, on a scale of the same 

 size as that which they are intended to have at all 

 future periods ; because, as we have just seen, after 

 the innermost layer has been deposited, they admit 

 not of any future enlargement of their cavity. 

 Thus we find that, in the case of polypes which 

 are lodged in cells, the walls of these cells must be 

 completed before the soft polypous portion has at- 

 tained its full expansion ; for were it at first built 

 of a smaller size, proportioned to that of the young 

 polype, it would prevent all further growth. 



The globular shell of the Echinus, which is ex- 

 ternal to the soft parts that nourish it, and which 

 yet grows from a very minute sphere to one of 

 large dimensions, keeping pace with the gradual 

 expansion of the internal organs, might appear to 

 be an exception to the general law. Nature has, 

 however, accomplished her purpose without deviat- 



