152 THE MECHANICAL FUNCTIONS. 



more rapid; while the sand, that had been pushed aside, 

 flows back, and covers the body, when it has sunk below 

 the level of the surface. In this situation the long dorsal 

 bristles come into play, preventing the sand from closing 

 completely, and preserving a small round hole for the ad- 

 mission of water to the mouth and respiratory organs.* 



Whenever, in following the series of organic structures, 

 new forms are met with, we always find them accompanied 

 by corresponding modifications in the processes of develop- 

 ment. The organization of the animals belonging to the 

 lowest division of the series is not sufiiciently perfect to af- 

 ford the means, which are supplied in the higher animals, 

 of removing or modifying the substances that have at any 

 time been deposited, and sufiered to harden. Hence the 

 structures composed of these substances remain unchanged 

 during the life-time of the animal, although they may conti- 

 nue 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 struc- 

 tures 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 relation to the 

 conditions of growth, and to the forms which result, it will 

 be necessary to direct our attention to them more particu- 

 larly. 



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 w^iich it is 

 to desposite 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 



The account here given is taken from Mr. Osier's papers in the Philosophi- 

 cal Transactions for 1826, p. 347. 



