314 TUBIPORA MUSICA. CILIOBRACH1ATA. 



inity with each other, and form this collar at about the same 

 time, an almost continuous horizontal partition or floor is thus 

 constructed, which gives great additional strength to these 

 delicate polypidoms. This collar is produced in the following 

 manner. The membranous continuation of the tube, instead of 

 growing straight upwards, makes a turn outwards, as if it were 

 flattened down all round. A double fold is thus occasioned, in 

 which calcareous matter is deposited, and the collar is thus com- 

 pleted. From this point the membrane is prolonged in a straight 

 direction as before, until the new impulse arises, which causes 

 another floor to be constructed ; and thus a succession of stories 

 is built up. The ova, when they issue from the parent, have 

 little or nothing of their perfect form. They seem to fall upon 

 a neighbouring portion of the floor, and there to begin the de- 

 velopment of a tube, which grows up among the older ones. Thus 

 it happens that, between every two floors, there are more tubes 

 than in the division below ; and the whole mass assumes some- 

 what of the form of an inverted pyramid. 



1161. The most extended survey we can take of the opera- 

 tions of the Polypifera upon the surface of the globe at the 

 present time, will give us but a very inadequate idea of the 

 important part which they performed, in the remoter epochs of 

 the history of the earth. Our wonder is excited when we hear 

 of a continuous reef of coral more than a thousand- miles in 

 length; yet what is this to the formation of limestone strata, 

 covering superficial areas, not only of thousands, but of tens of 

 thousands, of square miles, to a thickness, in many instances, of 

 3000 feet ? Yet the Geologist of the present day has little hesi- 

 tation in regarding these formations, as having taken their origin 

 from the labours of these apparently insignificant and simply- 

 organised beings. As at the present time, the greater proportion 

 of these structures appears to have been composed of the La- 

 melliform corals ( 1069) ; but the remains of Alcyonian Polypes 

 are by no means unfrequent in the limestone rocks, and are 

 especially abundant in particular strata. 



1162. There are many instances in which the Coral struc-* 

 tures of comparatively recent origin have undergone a metamor- 



