412 



RADIATA. 



surface from a great depth, the action of the Polype 

 is confined to the encrusting over, pretty thickly 

 it may be allowed, steep rocks previously existing 

 there. Submarine volcanoes may occasionally lift a 

 broad surface of the ocean-bottom, on which thick 

 layers of madrepores exist, and wherever this forma- 

 tion is found above the level of the tide, this must 

 undoubtedly have been the case. 



In some of this Order, the connecting substance 

 is of a fleshy texture, without any stony centre, as in 

 Cydonium.* This consists of an oblong, somewhat 

 rounded mass, affixed to the rocks by a broad flat 

 base, of a soft, but somewhat gristly substance. The 

 surface is studded with little celL, in which are 

 lodged minute Polypes with eight tentacles. The 

 whole mass is capable of contraction and dilatation. 



The coralline skeleton assumes a singular form 

 in the Sea-Pens (Pennatula), consisting of a length- 

 ened stem or axis, supporting a double series of 

 jointed branches, on which the Polypes are arranged. 

 The whole form is that of a feather; and as it is 

 not fixed to any other substance, it has been sup- 

 posed to have the power of swimming in the sea, 

 by the motion of the branches ; but this fact is not 

 sufficiently established. 



, kydonion, a quince. 



