CILIOBRACHIATE POLYPES ; PLITSTRA. 533 



that the cells in the centre of a leaf-like expansion of the Flustra 

 are devoid of polypes, and have lost their vitality, whilst the 

 edges are in a state of active growth. Although this mode of 

 multiplication appears to be the one most characteristic of the 

 group, the production of gemmules or ova occasionally takes 

 place, as in other Polypifera. These are developed from the 

 lining membrane of the cell, and gradually increase so as to fill 

 up its cavity, and to cause the death of the contained polype. 

 When mature, they escape from the cell, and swim about for 

 some time, after which they fix themselves, and lay the founda- 

 tion of new structures. 



1105. The polypes forming these compound structures are 

 usually packed closely together ; and are of very minute size ; 



c b a 



Vm. C-25. PLUMATELLA : a, natural size ; 6, a group enlarged ; c, anal orifice. 



so that a single polypidom of very moderate dimensions must 

 contain an enormous multitude of individuals. Dr. Grant has 

 reckoned that, on an ordinary specimen of Flustra carlasea, there 

 are about ten square inches of surface ; in each square inch there 

 may be about 1800 polype-cells, thus making altogether 18,000 

 within this small space. Each of these polypes has 22 tenta- 

 cula ; so that there will be about 396,000 of these minute arms 



