THE EVOLUTION OF CONTINUITY 165 



explain how further growth had to be through gemmation, 

 equilibrium of personality of the first zooid having been 

 attained. 



The following diagrams are intended to illustrate some 

 of the points above suggested. 



_3333SCGOa_ 



Stu bi<L- . 



Fig. 47. — Illustrating the possible manner of evolution of the 

 tubular zooid. 1. Showing how a growing mass of cells of 

 similar density would, if the cells were of higher specific gravity 

 than the covering water, grow along the bottom as a flat mass. 

 2. Showing that if each cell produced gradually diminished in 

 density the cell -mass would as it grew gradually take a spherical 

 shape in presence of water-pressure, to become hollow in time 

 as in 3. The cell-mass is attached to the bottom. 4. As 

 growth proceeded diminishing cell density would cause the 

 sphere to elongate upwards towards the surface ; and a point 

 would be reached, as in 5, when the cells of the distal end of 

 the tube would give way owing to the upward pull, possibly 

 assisted by increasing internal pressure in the tube, and a 

 mouth be formed. Further growth would be from the margins 

 of the mouth, and owing to still diminishing cell-density would 

 be in the form of discrete series of cells, or primitive tentacles, 6. 



It is natural to suppose that any daughter-zooid budded 

 by a " parent "-zooid would as a whole be of lower density 

 than its producer, for it would originate from cells of lower 

 density than the first cells of the parent ; and it may be that 

 the final discontinuity of parent and daughter, as exemplified 

 in Hydra, was originally due mainly to the pull upwards of 

 the lighter budded zooid. 



In Continuously Zooidal Individuals, the zooids remain 

 attached, and there is zooidal arrest. It is not easy to 

 imagine how this new Continuity- variation came about, but 

 it may be noted that in modern zooidal colonies there is a 

 branching ccenosarc from which gemmation takes place, 

 and that a tough horny periderm is usually present, giving 

 the colony at times a coralline appearance. Permanent 

 attachment of the zooids might have originated as a result 



