526 TTTBIPORA MUSICA. CILIOBRACHIATA. 



the tube, within which the Polype can be entirely withdrawn. 

 The tube is gradually prolonged by the deposition of stony mat- 

 ter in this membrane, which is continuous with its upper edge ; 

 and the Polype always maintains its position at its extremity. 

 At intervals, however, a sort of collar is formed around each 

 tube ; and as a number of Polypes usually grow in close proxi- 

 mity 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 deve- 

 lopment 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. 



ORDER IV. CILIOBRACHIATA. 



1098. The last order of POLYPIFERA differs so greatly from 

 all the rest in its internal organisation, that it almost deserves to 

 rank as a separate class. As the Asteroida connect the Polypes 

 with the Sponges, so does this group form the transition to classes 

 of much higher organisation. In general aspect and delicacy of 

 structure, the Polypes of this order bear no inconsiderable re- 

 semblance to the compound Hydraform groups ( 1052) ; and 

 especially to the SertularidcK^ under which family many of them 

 have until recently been classed. They are not known to occur 



