ECHINUS. 153 



which must sooner or later become sensible. If the soft 

 organs have sufBcient 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 pre- 

 served unaltered. But this cannot happen when the new 

 materials are to be deposited on the internal surface of a 

 membrane, or a shell, which completely 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 lodgement of soft organs, are to be constructed of hard 

 materials, that the foundation of these cells should be laid, 

 and their construction begun, upon 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 polypoMS portion has attained its 

 full expansion; for were it at first built of a smaller size, 

 proportioned to that of the young polype, it would prevent 

 all farther growth. 



The globular shell of the Echinus, which is external to 

 the soft parts that nourish it, and which yet grows from a 

 very minute sphere to one of large dim.ensions, kee])ing pace 

 with the gradual expansion of the internal organs, might ap- 

 pear to be an exception to the general law. Nature has, 

 however, accomplished her purpose without deviating from 

 her usual plan; first, by dividing the shell of the Echinus into 

 a great number of small pieces; and secondly, by giving to 

 each piece the polygonal form, which is best adapted to their 



Vol. I. 20 



